[media] s2255drv: refactoring s2255_channel to s2255_vc

Renaming s2255_channel and all instances of channel to vc (video channel).

Signed-off-by: Dean Anderson <linux-dev@sensoray.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>