mfd: Use completion interrupt for WM831x AUXADC

Use the completion interrupt generated by the device rather than
polling for conversions to complete. As a backup we still check
the status of the AUXADC if we don't get a completion, mostly for
systems that don't have the WM831x interrupt infrastructure hooked
up.

Also reduce the timeout for completion of conversions to 5ms from
the previous 10ms, the lower timeout should be sufficient.

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