[media] si2157: change firmware download error handling

Rename firmare download error path goto label. Remove firmware NULL
set as NULL value is not needed anymore, due to recent change which
started using goto labels for firmware error handling.

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