AP_AHRS: correct get_accel() to use primary accel rather than first u…

…sable for scripting

libraries/AP_AHRS/AP_AHRS.h

@@ -620,7 +620,7 @@ class AP_AHRS {
 
     // return primary accels, for lua
     const Vector3f &get_accel(void) const {
-        return AP::ins().get_accel();
+        return AP::ins().get_accel(_get_primary_accel_index());
     }
 
     // return primary accel bias. This should be subtracted from