Copter: Move CH7 - CH8 check from read_aux_switches() to pre_arm_checks.

Moving this to pre-arm checks save some processing time and is safer.

ArduCopter/control_modes.pde

@@ -61,11 +61,6 @@ static void read_aux_switches()
         do_aux_switch_function(g.ch7_option, ap_system.CH7_flag);
     }
 
-    // safety check to ensure we ch7 and ch8 have different functions
-    if (g.ch7_option == g.ch8_option) {
-        return;
-    }
-
     // check if ch8 switch has changed position
     if (ap_system.CH8_flag != (g.rc_8.radio_in >= AUX_SWITCH_PWM_TRIGGER)) {
         // set the ch8 flag

ArduCopter/motors.pde

@@ -221,6 +221,14 @@ static void pre_arm_checks(bool display_failure)
         }
         return;
     }
+    
+    // pre-arm check to ensure ch7 and ch8 have different functions
+    if (g.ch7_option == g.ch8_option) {
+        if (display_failure) {
+            gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Check CH7 and CH8 settings"));
+        }
+        return;
+    }
 
     // check accelerometers have been calibrated
     if(!ins.calibrated()) {