[media] si2157: simplify si2157_cmd_execute() error path

Remove if () from firmware command error path as there should not be
any error prone conditional logic there. Use goto labels instead.

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