ASoC: wm8904: Rely entirely on the core for bias level management

Even though the WM8904 is able to use idle_bias_off during both probe
and resume we were needlessly leaving the device in standby mode. Instead
power the device down as soon as we've confirmed that we can talk to it
and don't manage the bias level at all over suspend and resume, the core
will take us down to our minimum power level.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>