[media] si2157: rename device state variable from 's' to 'dev'

'dev' is likely most common name in kernel for structure containing
device state instance, so rename it in order to keep things
consistent.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>