extcon: arizona: Check we report a valid impedance

Occasionally we can trigger an interrupt before we have completed
impedance measurement, although the valid bit will still be set. This
patch spins reading the impedance value until a valid value is seen.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>