mmc: cavium: Check pointer before de-reference

Add a pointer check to prevent this smatch warning:

drivers/mmc/host/cavium.c:803 cvm_mmc_request()
	error: we previously assumed 'cmd->data' could be null (see line 782)

This is a theoretical fix because MMC_CMD_ADTC seems to imply
that cmd->data is not null. Nevertheless checking cmd->data
before using it improves readability.
Signed-off-by: NJan Glauber <jglauber@cavium.com>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

drivers/mmc/host/cavium.c

@@ -799,7 +799,7 @@ static void cvm_mmc_request(struct mmc_host *mmc, struct mmc_request *mrq)
 		  FIELD_PREP(MIO_EMM_CMD_IDX, cmd->opcode) |
 		  FIELD_PREP(MIO_EMM_CMD_ARG, cmd->arg);
 	set_bus_id(&emm_cmd, slot->bus_id);
-	if (mmc_cmd_type(cmd) == MMC_CMD_ADTC)
+	if (cmd->data && mmc_cmd_type(cmd) == MMC_CMD_ADTC)
 		emm_cmd |= FIELD_PREP(MIO_EMM_CMD_OFFSET,
 				64 - ((cmd->data->blocks * cmd->data->blksz) / 8));