[media] si2157: 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 is.

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