isdn: mISDN: Fix sleeping function called from invalid context

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

isdn: mISDN: Fix sleeping function called from invalid context

The driver can call card->isac.release() function from an atomic
context.

Fix this by calling this function after releasing the lock.

The following log reveals it:

[ 44.168226 ] BUG: sleeping function called from invalid context at kernel/workqueue.c:3018
[ 44.168941 ] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 5475, name: modprobe
[ 44.169574 ] INFO: lockdep is turned off.
[ 44.169899 ] irq event stamp: 0
[ 44.170160 ] hardirqs last enabled at (0): [<0000000000000000>] 0x0
[ 44.170627 ] hardirqs last disabled at (0): [<ffffffff814209ed>] copy_process+0x132d/0x3e00
[ 44.171240 ] softirqs last enabled at (0): [<ffffffff81420a1a>] copy_process+0x135a/0x3e00
[ 44.171852 ] softirqs last disabled at (0): [<0000000000000000>] 0x0
[ 44.172318 ] Preemption disabled at:
[ 44.172320 ] [<ffffffffa009b0a9>] nj_release+0x69/0x500 [netjet]
[ 44.174441 ] Call Trace:
[ 44.174630 ] dump_stack_lvl+0xa8/0xd1
[ 44.174912 ] dump_stack+0x15/0x17
[ 44.175166 ] ___might_sleep+0x3a2/0x510
[ 44.175459 ] ? nj_release+0x69/0x500 [netjet]
[ 44.175791 ] __might_sleep+0x82/0xe0
[ 44.176063 ] ? start_flush_work+0x20/0x7b0
[ 44.176375 ] start_flush_work+0x33/0x7b0
[ 44.176672 ] ? trace_irq_enable_rcuidle+0x85/0x170
[ 44.177034 ] ? kasan_quarantine_put+0xaa/0x1f0
[ 44.177372 ] ? kasan_quarantine_put+0xaa/0x1f0
[ 44.177711 ] __flush_work+0x11a/0x1a0
[ 44.177991 ] ? flush_work+0x20/0x20
[ 44.178257 ] ? lock_release+0x13c/0x8f0
[ 44.178550 ] ? __kasan_check_write+0x14/0x20
[ 44.178872 ] ? do_raw_spin_lock+0x148/0x360
[ 44.179187 ] ? read_lock_is_recursive+0x20/0x20
[ 44.179530 ] ? __kasan_check_read+0x11/0x20
[ 44.179846 ] ? do_raw_spin_unlock+0x55/0x900
[ 44.180168 ] ? ____kasan_slab_free+0x116/0x140
[ 44.180505 ] ? _raw_spin_unlock_irqrestore+0x41/0x60
[ 44.180878 ] ? skb_queue_purge+0x1a3/0x1c0
[ 44.181189 ] ? kfree+0x13e/0x290
[ 44.181438 ] flush_work+0x17/0x20
[ 44.181695 ] mISDN_freedchannel+0xe8/0x100
[ 44.182006 ] isac_release+0x210/0x260 [mISDNipac]
[ 44.182366 ] nj_release+0xf6/0x500 [netjet]
[ 44.182685 ] nj_remove+0x48/0x70 [netjet]
[ 44.182989 ] pci_device_remove+0xa9/0x250