[media] s2255drv: Remove redundant NULL test before release_firmware()

release_firmware() tests for NULL pointers on its own - there's no
reason to do an explicit check before calling the function.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>