media: tuner-simple: allow setting mono radio mode

For some types of tuners (Philips FMD1216ME(X) MK3 currently) we know that
letting TDA9887 output port 1 remain high (inactive) will switch FM radio
to mono mode.
Let's make use of this functionality - nothing changes for the default
stereo radio mode.

Tested on a Medion 95700 board which has a FMD1216ME tuner.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>