[media] af9013: Fix typo in get_frontend() function

This patch fixes an obvious typo in the get_frontend() function
of the af9013 driver, recently rewritten by Antti Palosaari.

Signed-off-by: Gianluca Gennari <gennarone@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>