V4L/DVB (9876): gspca - main: Allow subdrivers to handle v4l2_streamparm requests.

Add get_streamparm and set_streamparm operations so subdrivers can
get/set stream parameters such as framerate.

Signed-off-by: Jim Paris <jim@jtan.com>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>