AP_Compass: fix compilation issue with declaring var after goto

libraries/AP_Compass/AP_Compass_RM3100.cpp

@@ -212,14 +212,13 @@ void AP_Compass_RM3100::timer()
     // some RM3100 builds get the polarity wrong on one or more of the
     // elements. By setting AP_RM3100_REVERSAL_MASK in hwdef.dat you
     // can fix it without modifying the hardware
-    const uint8_t mask = AP_RM3100_REVERSAL_MASK;
-    if (mask & 1U) {
+    if (AP_RM3100_REVERSAL_MASK & 1U) {
         magx = -magx;
     }
-    if (mask & 2U) {
+    if (AP_RM3100_REVERSAL_MASK & 2U) {
         magy = -magy;
     }
-    if (mask & 4U) {
+    if (AP_RM3100_REVERSAL_MASK & 4U) {
         magz = -magz;
     }
 #endif