AP_Motors: fixed PWM_TYPE range for quadplanes

when motors don't start at SERVO1 we were applying the range to the
wrong output.

This allows users to control the PWM of individual motors precisely
for quadplanes where calibration of motors is difficult

libraries/AP_Motors/AP_Motors_Class.cpp

@@ -112,13 +112,13 @@ void AP_Motors::rc_write_angle(uint8_t chan, int16_t angle_cd)
 /*
   set frequency of a set of channels
  */
-void AP_Motors::rc_set_freq(uint32_t mask, uint16_t freq_hz)
+void AP_Motors::rc_set_freq(uint32_t motor_mask, uint16_t freq_hz)
 {
     if (freq_hz > 50) {
-        _motor_fast_mask |= mask;
+        _motor_fast_mask |= motor_mask;
     }
 
-    mask = motor_mask_to_srv_channel_mask(mask);
+    const uint32_t mask = motor_mask_to_srv_channel_mask(motor_mask);
     hal.rcout->set_freq(mask, freq_hz);
 
     switch (pwm_type(_pwm_type.get())) {
@@ -152,9 +152,9 @@ void AP_Motors::rc_set_freq(uint32_t mask, uint16_t freq_hz)
           this is a motor output type for multirotors which honours
           the SERVOn_MIN/MAX values per channel
          */
-        _motor_pwm_range_mask |= mask;
+        _motor_pwm_range_mask |= motor_mask;
         for (uint8_t i=0; i<16; i++) {
-            if ((1U<<i) & mask) {
+            if ((1U<<i) & motor_mask) {
                 SRV_Channels::set_range(SRV_Channels::get_motor_function(i), 1000);
             }
         }