mmc: core: Replacing sprintf() with snprintf()

While using the sysfs entry for getting the different bit values
of EXT_CSD register, it is returned by sprintf(). But sprintf() has
been deprecated, that's why replace with safe function snprintf().
By using this patch, PAGE_SIZE(4Kb) is the max number of characters
store in a buffer.

Change-Id: Ifb3dc39e8d3728094fff10c7e761dc19fdfaa854
Signed-off-by: Siba Prasad <sibap@codeaurora.org>