arm: use symbolic MDCR_TDE in arm_debug_target_el

We already have this symbol defined so lets use it.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20181109152119.9242-7-alex.bennee@linaro.org
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/arm/cpu.h

@@ -2743,7 +2743,7 @@ static inline int arm_debug_target_el(CPUARMState *env)
 
     if (arm_feature(env, ARM_FEATURE_EL2) && !secure) {
         route_to_el2 = env->cp15.hcr_el2 & HCR_TGE ||
-                       env->cp15.mdcr_el2 & (1 << 8);
+                       env->cp15.mdcr_el2 & MDCR_TDE;
     }
 
     if (route_to_el2) {