[media] redrat3: change return argument on redrat3_send_cmd() to int

redrat3_send_cmd() can return an error or the read data. However,
it currently returns an u8, as reported by smatch:
	drivers/media/rc/redrat3.c:416 redrat3_send_cmd() warn: signedness bug returning '(-12)'

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>