hwmon: (adt7470) Add write support to alarm_mask

Add write support for the alarm_mask. A base of 0 is provided so that
either hex or decimal can be used. The hex format when reading alarm_mask
is unchanged.

Signed-off-by: Joshua Scott <joshua.scott@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>