[media] af9015: various small changes and clean-ups

Clean-up dvb_usb_device_properties and fix errors
reported by checkpatch.pl.

Some other very minor changes. Functionality remains
untouched.
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>