[media] si2157: rework firmware download logic a little bit

Rework firmware selection / chip detection logic a little bit.
Add missing release_firmware() to error path.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Tested-by: Olli Salonen <olli.salonen@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>