[media] gspca: Allow subdrivers to use the control framework

Make the necessary changes to allow subdrivers to use the control framework.
This does not add control event support, that comes later.

It add a init_control cam_op that is called after init in probe that allows
the subdriver to set up the controls.

HdG: Call v4l2_ctrl_handler_setup from resume instead of
gspca_set_default_mode, as we just want to resend the current ctrl values to
the device.

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