gspca_sn9c20x: Fix NULL pointer dereference

Don't call v4l2_ctrl_g_ctrl on ctrls which the model cam in question
does not have.

Reported-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
[ Taken directly, since Mauro is on vacation ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>