ASoC: cx20442: add bias control over a platform provided regulator

Now that a regulator device for controlling the codec chip reset state
over a platform agnostic regulator API is available on the only board
using this driver so far, extend the driver with a bias control function
which will request virtual power to the codec chip from that virtual
regulator, and will supersede the present implementation existing at the
sound card level.

Thanks to the regulator sharing mechanism, both the old (the sound card)
and the new (the codec) implementations should coexist smoothly until
the sound card file is updated. For this to work as expected, update the
sound card .set_bias_level callback to not touch codec->dapm.bias_level.

While extending the cx20442 structure, drop unused control_type member.

Created against linxu-3.2-rc6, tested on top of patch 1/4 "ARM: OMAP1:
ams-delta: set up a regulator over the modem reset GPIO pin".

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@ti.com>