mmc: debugfs: fix unclocked register access by force_error

When echo values to debugfs entry force_error, unclocked
register access error would be detected. This change fixes
it by making sure the host is powered up and host clock is
ticking before access force event register for error
interrupt status.

Change-Id: I995523d64de36cd4872209db4a558aa83103fbb3
Signed-off-by: Can Guo <cang@codeaurora.org>