[media] DVB-frontends: Deletion of unnecessary checks before the function call "release_firmware"

GIT_AUTHOR_DATE=1416472432
The release_firmware() function tests whether its argument is NULL
and then returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

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