mfd: Use the value of the final spin when reading the AUXADC

Reverse the order of the tests for loop exit so we use a valid value
before we time out.  Vanishingly unlikely to happen since we retry for
several times the expected conversion time.

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