[media] gspca - main: Correct use of interval in bandwidth calculation

The calculated bandwidth should not be multiplied by the interval, but be
divided by it. Also bInterbval should be interpreted as a power of 2
for isochronous endpoints.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>