[media] gspca-stv06xx: support bandwidth changing

stv06xx devices have only one altsetting, but the actual used
bandwidth can be programmed through a register. We were already
setting this register lower then the max packetsize of the altsetting
indicates. This patch makes the gspca-stv06xx update the usb descriptor
for the alt setting to reflect the actual packetsize in use, so that
the usb subsystem uses the correct information for scheduling usb transfers.

This patch also tries to fallback to lower speeds in case a ENOSPC error
is received when submitting urbs, but currently this is only supported
with stv06xx cams with the pb0100 sensor, as this is the only one for
which we know how to change the framerate.

This patch is based on an initial incomplete patch by
Lee Jones <lee.jones@canonical.com>

Signed-off-by: Lee Jones <lee.jones@canonical.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>