hwmon: (max6650) Read non-volatile registers only once

Only tachometer and alarm status registers are modified by the chip.
All other registers only need to be read only once, and reading them
repeatedly does not add any value.

Cc: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>