mfd: Use completion interrupt for WM835x AUXADC

Use the completion interrupt generated by the device rather than
polling for conversions to complete. As a backup we still check
the state of the AUXADC if we don't get a completion, mostly for
systems that don't have the WM8350 interrupt infrastructure hooked
up.

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