[media] em28xx: fix usb alternate setting for analog and digital video endpoints > 0

The current code assumes that the analog + digital video endpoints are always at
interface number 0 when changing the alternate setting.
This seems to work fine for most existing devices.
However, at least the SpeedLink VAD Laplace webcam has the video endpoint on
interface number 3 (which fortunately doesn't cause any trouble because ist uses
bulk transfers only).
We already consider the actual interface number for audio endpoints, so
rename the the audio_ifnum variable and use it for all device types.
Also get get rid of a pointless (ifnum < 0) in em28xx-audio.

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