[media] si2168: remove unneeded fw variable initialization

commit 034e1ec0
si2168: One function call less in si2168_init() after error detection

That commit added goto label for error path to release firmware,
but forgets to remove variable NULL set. Remove those now.

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