[media] gspca core: Fix regressions gspca breaking devices with audio

Changeset 35680baa broke several devices:
	- Sony Playstation Eye (1415:2000);
	- Gigaware model 25-234 (0c45:628f);
	- Logitech Messenger Plus (046d:08f6).

Probably more devices were broken by this change.

What happens is that several devices don't need to save some bandwidth
for audio.

Also, as pointed by Hans de Goede <hdegoede@redhat.com>, the logic
that implements the bandwidth reservation for audio is broken, since
it will reduce the alt number twice, on devices with audio.

So, let's just revert the broken logic, and think on a better solution
for usb 1.1 devices with audio that can't use the maximum packetsize.

Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>