hwmon: (w83795) Expose fan control method

Expose fan control method (DC vs. PWM) using the standard sysfs
attributes. I've made it read-only as the board should be wired for
a given mode, the BIOS should have set up the chip for this mode, and
you shouldn't have to change it. But it would be easy enough to make
it changeable if someone comes up with a use case.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Guenter Roeck <guenter.roeck@ericsson.com>