[media] gspca: Fix bulk mode cameras no longer working (regression fix)

The new iso bandwidth calculation code accidentally has broken support
for bulk mode cameras. This has broken the following drivers:
finepix, jeilinj, ovfx2, ov534, ov534_9, se401, sq905, sq905c, sq930x,
stv0680, vicam.

Thix patch fixes this. Fix tested with: se401, sq905, sq905c, stv0680 &
vicam cams.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>