After some tedious work with a logic probe and a magnifying glass, I've
determined that GPIO 7 is used to switch between the DTT7612's Sound 4.5 MHz
IF output on pin 12 and the FM 10.7MHz If output on pin 11.  GPIO 2 is used to
switch the card's analog sound output from from the analog input connector to
the CX23883's audio DACs.

So, in radio mode GPIO2 = 1 and GPIO7 = 0.

Add some comments about how the HD-3000's GPIOs are connected.

Delete the vmux setting for the radio, as vmux doesn't apply to radio mode.
Also delete the lines setting unused gpio words to zero; it's not necessary as
0 is the default value for uninitialized fields.
Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>