V4L/DVB (11758): 2: handle unregister for non-I2C builds

Build fails when CONFIG_I2C=n, so handle that case in the if block:

drivers/built-in.o: In function `v4l2_device_unregister':
(.text+0x157821): undefined reference to `i2c_unregister_device'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>