[media] si2168: change firmware variable name and type

Rename firmware variable from fw_file to fw_name and change its type
from u8 to const char as request_firmware() input defines.

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