[media] gspca_gl860: Add a present check to sd_stop0

The sensor specific dev_post_unset_alt functions all try to write to the
bridge, and none free any memory, so they should be skipped if stop0
is called on disconnection.

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