[media] em28xx: fix check for audio only usb interfaces when changing the usb alternate setting

Previously, we've been assuming that the video endpoints are always at usb
interface 0. Hence, if vendor audio endpoints are provided at a separate
interface, they were supposed to be at interface number > 0.
Instead of checking for (interface number > 0) to determine if an interface
is a pure audio interface, dev->is_audio_only should be checked.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>