nvic: Add missing 'break'

Coverity points out that we forgot the 'break' for
the SAU_CTRL write case (CID1381683). This has
no actual visible consequences because it happens
that the following case is effectively a no-op.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 1507742676-9908-1-git-send-email-peter.maydell@linaro.org
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>

hw/intc/armv7m_nvic.c

@@ -1447,6 +1447,7 @@ static void nvic_writel(NVICState *s, uint32_t offset, uint32_t value,
             return;
         }
         cpu->env.sau.ctrl = value & 3;
+        break;
     case 0xdd4: /* SAU_TYPE */
         if (!arm_feature(&cpu->env, ARM_FEATURE_V8)) {
             goto bad_offset;