hwmon: (pwm-fan) split __set_pwm into locked/unlocked functions

Regular calls to set_pwm don't hold the mutex, but the upcoming
update_enable support needs to call set_pwm with the mutex being held.
So provide the previous behavior in set_pwm (handling the lock), while
adding __set_pwm which assumes the lock being held.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220914153137.613982-5-alexander.stein@ew.tq-group.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>