[media] si2157: add own goto label for kfree() on probe error

Use own goto label for error case mem free is needed, even kfree could
be called with NULL. I think it is better to have it, even not required.

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