[media] gspca: provide a mechanism to select a specific transfer endpoint

Currently gspca selects the first ISOC input endpoint as the input
transfer endpoint, however some devices can provide streams on endpoints
different then the first one, so some subdrivers (e.g. gspca_kinect) may
want to select a specific endpoint to use as a transfer endpoint.

Add an xfer_ep field to struct gspca_dev, and change alt_xfer() so that
it accepts a parameter which represents a specific endpoint address to
look for.

If a subdriver wants to specify a value for gspca_dev->xfer_ep it can do
that in its sd_config() callback.

Signed-off-by: Antonio Ospite <ao2@ao2.it>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>