[media] si2168: One function call less in si2168_init() after error detection

GIT_AUTHOR_DATE=1416472767
The release_firmware() function was called in some cases by the
si2168_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>