[media] af9013: Fix a compilation warning

drivers/media/dvb/frontends/af9013.c: In function ‘af9013_update_signal_strength.clone.0’:
drivers/media/dvb/frontends/af9013.c:967:6: warning: ‘ret’ may be used uninitialized in this function

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