hwmon: (smsc47m1) Enable device if needed

If the address is set but the device isn't enabled, attempt to enable
it. If it won't work for any reason (resource conflict, no function
enabled) the initial state is restored. The initial state is also
restored on module unloading.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Tested-by: Sean Fidler <fidlersean@gmail.com>