mmc: core: Return error if fallback to lower speed mode fails

If there are continuous data-CRC errors in higher speed modes (SDR104
mode), then driver fallbacks to lower speed mode. But if at all it
fails to fallback to lower speed mode, then that error should be
propagated to the caller so that caller will handle it appropriately.

Without this change, sometime while processing card removal event,
driver fails to detect card removal and treats card as present
eventhough its removed.

Change-Id: I89544d41c5b014eb9227ba33ef9ec1917b6793dc
Signed-off-by: Veerabhadrarao Badiganti <vbadigan@codeaurora.org>