Copter: use disparity threshold define for pre-arm checks

There are two duplicate checks, one in the pre-arm checks (i.e. checks
run every 15 seconds or so before the vehicle is armed) and one in the
arming checks (run immediately before arming).  The definition in the
pre-arm checks was still using the old hardcoded value.

ArduCopter/motors.pde

@@ -243,7 +243,7 @@ static void pre_arm_checks(bool display_failure)
             return;
         }
         // check Baro & inav alt are within 1m
-        if(fabs(inertial_nav.get_altitude() - baro_alt) > 100) {
+        if(fabs(inertial_nav.get_altitude() - baro_alt) > PREARM_MAX_ALT_DISPARITY_CM) {
             if (display_failure) {
                 gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Alt disparity"));
             }