Bluetooth: btnxpuart: Fix Null pointer dereference in btnxpuart_flush()

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: btnxpuart: Fix Null pointer dereference in btnxpuart_flush()

This adds a check before freeing the rx->skb in flush and close
functions to handle the kernel crash seen while removing driver after FW
download fails or before FW download completes.

dmesg log:
[ 54.634586] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000080
[ 54.643398] Mem abort info:
[ 54.646204] ESR = 0x0000000096000004
[ 54.649964] EC = 0x25: DABT (current EL), IL = 32 bits
[ 54.655286] SET = 0, FnV = 0
[ 54.658348] EA = 0, S1PTW = 0
[ 54.661498] FSC = 0x04: level 0 translation fault
[ 54.666391] Data abort info:
[ 54.669273] ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[ 54.674768] CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[ 54.674771] GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[ 54.674775] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000048860000
[ 54.674780] [0000000000000080] pgd=0000000000000000, p4d=0000000000000000
[ 54.703880] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[ 54.710152] Modules linked in: btnxpuart(-) overlay fsl_jr_uio caam_jr caamkeyblob_desc caamhash_desc caamalg_desc crypto_engine authenc libdes crct10dif_ce polyval_ce polyval_generic snd_soc_imx_spdif snd_soc_imx_card snd_soc_ak5558 snd_soc_ak4458 caam secvio error snd_soc_fsl_micfil snd_soc_fsl_spdif snd_soc_fsl_sai snd_soc_fsl_utils imx_pcm_dma gpio_ir_recv rc_core sch_fq_codel fuse
[ 54.744357] CPU: 3 PID: 72 Comm: kworker/u9:0 Not tainted 6.6.3-otbr-g128004619037 #2
[ 54.744364] Hardware name: FSL i.MX8MM EVK board (DT)
[ 54.744368] Workqueue: hci0 hci_power_on
[ 54.757244] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 54.757249] pc : kfree_skb_reason+0x18/0xb0
[ 54.772299] lr : btnxpuart_flush+0x40/0x58 [btnxpuart]
[ 54.782921] sp : ffff8000805ebca0
[ 54.782923] x29: ffff8000805ebca0 x28: ffffa5c6cf1869c0 x27: ffffa5c6cf186000
[ 54.782931] x26: ffff377b84852400 x25: ffff377b848523c0 x24: ffff377b845e7230
[ 54.782938] x23: ffffa5c6ce8dbe08 x22: ffffa5c6ceb65410 x21: 00000000ffffff92
[ 54.782945] x20: ffffa5c6ce8dbe98 x19: ffffffffffffffac x18: ffffffffffffffff
[ 54.807651] x17: 0000000000000000 x16: ffffa5c6ce2824ec x15: ffff8001005eb857
[ 54.821917] x14: 0000000000000000 x13: ffffa5c6cf1a02e0 x12: 0000000000000642
[ 54.821924] x11: 0000000000000040 x10: ffffa5c6cf19d690 x9 : ffffa5c6cf19d688
[ 54.821931] x8 : ffff377b86000028 x7 : 0000000000000000 x6 : 0000000000000000
[ 54.821938] x5 : ffff377b86000000 x4 : 0000000000000000 x3 : 0000000000000000
[ 54.843331] x2 : 0000000000000000 x1 : 0000000000000002 x0 : ffffffffffffffac
[ 54.857599] Call trace:
[ 54.857601] kfree_skb_reason+0x18/0xb0
[ 54.863878] btnxpuart_flush+0x40/0x58 [btnxpuart]
[ 54.863888] hci_dev_open_sync+0x3a8/0xa04
[ 54.872773] hci_power_on+0x54/0x2e4
[ 54.881832] process_one_work+0x138/0x260
[ 54.881842] worker_thread+0x32c/0x438
[ 54.881847] kthread+0x118/0x11c
[ 54.881853] ret_from_fork+0x10/0x20
[ 54.896406] Code: a9be7bfd 910003fd f9000bf3 aa0003f3 (b940d400)
[ 54.896410] ---[ end trace 0000000000000000 ]---