sd: sdhci: mask transfer mode register value

In SDHCI protocol, the transfer mode register is defined
to be of 6 bits. Mask its value with '0x0037' so that an
invalid value could not be assigned.
Signed-off-by: NPrasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: NAlistair Francis <alistair.francis@xilinx.com>
Message-id: 20170214185225.7994-2-ppandit@redhat.com
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

hw/sd/sdhci.c

@@ -119,6 +119,7 @@
     (SDHC_CAPAB_BASECLKFREQ << 8) | (SDHC_CAPAB_TOUNIT << 7) | \
     (SDHC_CAPAB_TOCLKFREQ))
 
+#define MASK_TRNMOD     0x0037
 #define MASKED_WRITE(reg, mask, val)  (reg = (reg & (mask)) | (val))
 
 static uint8_t sdhci_slotint(SDHCIState *s)
@@ -1050,7 +1051,7 @@ sdhci_write(void *opaque, hwaddr offset, uint64_t val, unsigned size)
         if (!(s->capareg & SDHC_CAN_DO_DMA)) {
             value &= ~SDHC_TRNS_DMA;
         }
-        MASKED_WRITE(s->trnmod, mask, value);
+        MASKED_WRITE(s->trnmod, mask, value & MASK_TRNMOD);
         MASKED_WRITE(s->cmdreg, mask >> 16, value >> 16);
 
         /* Writing to the upper byte of CMDREG triggers SD command generation */