V4L/DVB: gspca - ov534: Add Powerline Frequency control

Note that setting this options to 50Hz can reduce the framerate, so it is
still disabled (60Hz) by default.

Signed-off-by: Mosalam Ebrahimi <m.ebrahimi@ieee.org>
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>