mmc: card: fix null pointer deference in cmdq timeout handler

The mmc_queue_req will be present only if the request is issued
to the LLD. The request could be fetched from block layer queue
but could be waiting to be issued (for e.g. clock scaling is
waiting for an empty cmdq queue). Evaluate such cases and reset
the timer to give LLD more time.

Change-Id: Ic5818f5c2b8356bda9b1612d78b65e07dad011d7
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>