[media] pwc: Fix comment wrt lock ordering

With all the changes to handle the locking in the v4l2-core rather then at
the driver level, the order in which the 2 pwc locks need to be taken has
changed, update the comment in the header file to correctly reflect this.

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