switch: arizona: Take external resistance into account for short circuit

When checking the minimum value of impedance we can detect we should
take any external resistance into account, as we can detect lower
impedances if there is some external resistance pushing the detected
impedance up.

Change-Id: Ic784807c811ab453a7ca8ef2fb115d1bcd884e87
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>