hwmon: (lm85) Make the pwmN_enable files writable

Make the pwmN_enable files writable. This makes it possible to use
standard fan speed control tools (pwmconfig, fancontrol) with the lm85
driver.

I left the non-standard pwmN_auto_channels files in place, as they
give additional control for the automatic mode, and some users might
be used to them by now.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>