mmc: workaround for read ahead issue

When the sequential read is interrupted by a special operation (RPMB access,
Secure Erase and Secure Trim) the read ahead feature does not function as
intended. In that case, data from special operation is not removed from the
internal buffer. So when we read the next address, the reading data from the
previous operation instead of the address.

To avoid the issue, add dummy read and ignore the data in that case.

Change-Id: I6c1a3285034c9e057879de601e1f80a4bed07edb
Signed-off-by: Devin Kim <dojip.kim@lge.com>