hwmon: (w83627ehf) Optimize multi-bank register access

Assume that each register is banked, and set the bank for each access.
Cache the bank number so it only needs to be set if it changes.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Ian Dobson <i.dobson@planet-ian.com>