switch: arizona: Correct handling of external resistance for low values

For external resistance values < 4 ohms we might currently set a high
short circuit impedance than we have to. Correct this by taking the
external resistance into account.

Change-Id: Idbb68a39c5ee763284b8763d218af89353f74e6c
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>