extcon: arizona: Factor out microphone impedance into a function

The microphone detection handler is very long, start breaking it up
by factoring out the actual reading of the impedance value into a
separate functions. Additionally, this also fixes a minor bug and
ensures that the microphone timeout will be rescheduled in all error
cases.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>