[media] hackrf: add control for RF amplifier

There is Avago MGA-81563 amplifier just right after antenna connector.
It could be turned on/off and its gain is around 12dB.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>