mmc: mmc-debugfs: add error state

This change adds support to allow user space query if low level eMMC
driver has encountered any error or not, this state can be read/cleared
via debugfs.

CRs-Fixed: 1056483
Change-Id: Idc4ea375e9f308446dec04d443d062fe502658bd
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>