mfd: Clean up after WM83xx AUXADC interrupt if it arrives late

In certain circumstances, especially under heavy load, the AUXADC
completion interrupt may be detected after we've timed out waiting for
it.  That conversion would still succeed but the next conversion will
see the completion that was signalled by the interrupt for the previous
conversion and therefore not wait for the AUXADC conversion to run,
causing it to report failure.

Provide a simple, non-invasive cleanup by using try_wait_for_completion()
to ensure that the completion is not signalled before we wait.  Since
the AUXADC is run within a mutex we know there can only have been at
most one AUXADC interrupt outstanding.  A more involved change should
follow for the next merge window.

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