gspca:    Protect dq_callback() against simultaneous USB exchanges.
          Temporary buffer for USB exchanges added in the device struct.
(all)     Use a temporary buffer for all USB exchanges.
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>