Input: wm97xx - refactor channel selection in poll_sample()

The current implementation of poll_sample() has the problem that one of
its arguments, the channel to be selected, differs from wm9713 to other
variants. This parameter gets passed to the (currently unused)
mach-specific functions pre_sample() and post_sample() which thus have
to deal with codec-specific differences. Refactor the routine so that
the argument to poll_sample() is generic for all codecs and do necessary
conversions only in the codec-specific driver. The outcome even uses
less code and removes the non-standard use of the PEN_DOWN bit to mark
the AUX-channels.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>