media: cec: core: Fix kmemleak due to missed rc_free_device() call

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

media: cec: core: Fix kmemleak due to missed rc_free_device() call

The commit dccc0c3ddf8f ("media: rc: fix race between unregister and
urb/irq callbacks") removed the implicit call to rc_free_device() from
rc_unregister_device(). However, the commit missed to remove the NULL
assignment of adap->rc that is now causing rc_free_device() to never be
called on an allocated rc device.

kmemleak reports following after e.g. dw-hdmi unbind:

unreferenced object 0xffff00010ac10000 (size 4096):
comm "kworker/u16:1", pid 39, jiffies 4294897739
hex dump (first 32 bytes):
20 23 4b 0a 01 00 ff ff 08 00 c1 0a 01 00 ff ff #K.............
08 00 c1 0a 01 00 ff ff 00 00 00 00 00 00 00 00 ................
backtrace (crc e11baccc):
kmemleak_alloc+0x38/0x44
__kmalloc_cache_noprof+0x4a8/0x5e0
rc_allocate_device+0x48/0x2a0
cec_allocate_adapter+0x3ac/0x800
dw_hdmi_cec_probe+0x264/0x634
platform_probe+0xc0/0x188
really_probe+0x4a4/0x8e0
__driver_probe_device+0x2f8/0x440
driver_probe_device+0x60/0x160
__device_attach_driver+0x1a0/0x2a0
bus_for_each_drv+0x100/0x1a0
__device_attach+0x174/0x350
device_initial_probe+0x90/0xb0
bus_probe_device+0x4c/0x120
device_add+0xdec/0x116c
platform_device_add+0x354/0x598

Remove the assignment of adap->rc to NULL to let cec_delete_adapter()
free the allocated rc device after last user of the cec device exits to
fix the kmemleak.