[media] hackrf: add support for transmitter

HackRF SDR device has both receiver and transmitter. There is limitation
that receiver and transmitter cannot be used at the same time
(half-duplex operation). That patch implements transmitter support to
existing receiver only driver.

Signed-off-by: Antti Palosaari <crope@iki.fi>
[hans.verkuil@cisco.com: fix krobot unused variable warning]
[hans.verkuil@cisco.com: add back s_modulator from PATCHv4]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>