mfd: Improve WM831x AUXADC completion handling

Currently completion of WM831x AUXADC conversions is monitored by
checking for convertor enable. Due to the mechanism used to ensure
data corruption is avoided when reading AUXADC data there may under
heavy I/O be a window where this bit has cleared but the conversion
results have not been updated. Data availability is only guaranteed
after the AUXADC data interrupt has been asserted.

Avoid this by always using the interrupt to detect completion. If the
chip IRQ is not set up then we poll the IRQ status register for up to
5ms. If it is set up then we rely on the data done interrupt with a
vastly increased timeout, failing the conversion if the interrupt is
not generated.

This also saves a register read when using interrupts.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>