V4L/DVB (6185): cx88-alsa: Add mute controls, change control names

Add two mute controls.  One mutes everything, the other just mutes the analog
pass-through output.

Rename the existing volume control.  The controls are now:
Playback Volume
Playback Switch
Capture Switch

These names might seem odd, but I believe they are more correct.  The previous
"Capture Volume" control didn't actually effect the volume of the captured
audio.  Instead it controls the volume of the analog pass-thought output.  It
appears that pass-through controls like this are usually considered to be in
the playback direction, not capture.  For example, "CAPTURE feedback Playback
Volume" is the name used for a control that appears to have the same effect in
the ca0106 driver.  We only have one volume control, so we can omit the
"CAPTURE feedback" part.

If someone where to add PCM playback support to the driver, then this would be
the volume control.

Signed-off-by: Trent Piepho <xyzzy@speakeasy.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>