fan: Scale fan speed requests between 0 and max_power

If the fan's max power is limited by the config, then scale speed
requests between 0 and max_power.  This makes more sense for typical
g-code fan speeds.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -205,10 +205,10 @@
 #heater_temp: 50.0
 #   A temperature (in Celsius) that the heater must drop below before
 #   the fan is disabled. The default is 50 Celsius.
-#fan_speed:
+#fan_speed: 1.0
 #   The fan speed (expressed as a value from 0.0 to 1.0) that the fan
 #   will be set to when its associated heater is enabled. The default
-#   is max_power.
+#   is 1.0
 
 
 # Additional micro-controllers (one may define any number of sections

config/example.cfg

@@ -224,7 +224,10 @@ pin: ar9
 #   enabled for extended periods, while a value of 0.5 would allow the
 #   pin to be enabled for no more than half the time. This setting may
 #   be used to limit the total power output (over extended periods) to
-#   the fan. The default is 1.0.
+#   the fan. If this value is less than 1.0 then fan speed requests
+#   will be scaled between zero and max_power (for example, if
+#   max_power is .9 and a fan speed of 80% is requested then the fan
+#   power will be set to 72%). The default is 1.0.
 #cycle_time: 0.010
 #   The amount of time (in seconds) for each PWM power cycle to the
 #   fan. It is recommended this be 10 milliseconds or greater when