V4L/DVB (3577): Cleanup audio input handling

Cleanup audio input handling in bttv and tvaudio:
- inputs were specified that were never used
- mute was handled as a special input which led to confusing code
- confusing naming made it difficult to see if the setting was for
  i2c or gpio.
The old audiochip.h input names moved to tvaudio.h. Currently this
is used both by tvaudio and msp3400 until the msp3400 implements the
new msp3400-specific inputs.
Detect in bttv the tvaudio and msp3400 i2c clients and use these
client pointers to set the inputs directly instead of broadcasting the
command.
Removed AUDC_SET_INPUT. Now replaced by VIDIOC_S_AUDIO. This will be
replaced again later by the new ROUTING commands.
Removed VIDIOC_G_AUDIO implementations in i2c drivers: this command is
a user level command and not to be used internally. It wasn't called at
all anyway.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>