V4L/DVB (13138): gspca_sq905: cleanup sq905_dostream

-Remove use of unneeded discarding variable
-Cleanup locking to only take usb_lock around access to the control
 endpoint, by no longer taking the lock around the bulk transfer
 (which takes most of the time) we can remove the msleep(1) which was
 needed to give the gspca core a chance to grab the usb_lock to signal
 us to stop.

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