[media] si2157: One function call less in si2157_init() after error

The release_firmware() function was called in some cases by the si2157_init()
function during error handling even if the passed variable contained still
a null pointer. This implementation detail could be improved
by the introduction of another jump label.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>