mmc: Fix compile warnings

Fix the following compiler warning(s) from gcc 6.1.1 with -Werror enabled:
drivers/mmc/host/sdhci.c:4755:
  format '%s' expects a matching 'char *' argument [-Werror=format=]
  pr_info("%s: SDHCI controller on %s [%s] using %s in %s mode\n",

Change-Id: I7942b72ac9cbac240ff23b11dcbc3160bc112413
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>