[media] gspca_xirlink_cit: various usb bandwidth allocation improvements / fixes

The following usb bandwidth allocation changes were made to the ibm netcam
pro code:
- Don't restart negotiation at max packet size on stop0, as that gets called
  by gspca_main during negotiation. Move this to sd_isoc_init.
- Don't ask for full bandwidth when running at 160x120, that does not need
  full bandwidth
- Make minimum acceptable bandwidth depend upon resolution

[mchehab@redhat.com: Fix CodingStyle problems at switch statements]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>