mmc: Fix warning if CONFIG_MMC_TRACE is enabled

Fix the warning

mmc.c: In function 'mmc_send_cmd':
mmc.c:87: warning: assignment from incompatible pointer type

in case CONFIG_MMC_TRACE is enabled.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Andy Fleming <afleming@freescale.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>