[media] cx231xx: Remove a bogus check for NULL

As reported by kbuild test robot:
	drivers/media/usb/cx231xx/cx231xx-audio.c:445:16-20: ERROR: dev is NULL but dereferenced.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>