hwmon: (nct6775) Fix comment in the description of pwm_mode

Commit 415eb2a1 ("hwmon: (nct6775) Fix writing pwmX_mode") changed
the definition of the pwm_mode variable, but did not update the comment
describing it.

Reported-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Cc: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>