mmc: core: prevent calling ->post_req 2 times if request submit fails

Currently we might end up calling ->post_req() 2 times if we failed to
submit the request to host driver. This change fixes this issue.

Change-Id: I10b681ab955e0a9126b22df68d3601b75328c949
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>