mmc: core: ignore new request notification, when 2 requests are in progress

In case both host->areq and areq requests exist and the new request
notification unblocks mmc context, instead of BUG_ON() in mmc_start_req(),
mmc_wait_for_data_req_done() continue to wait again for the completion
of the current request. New request notification is ignored in this case.

Change-Id: Ie0f95ae7fb17a8cb7a62b3805d066593d967e279
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>