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: I5a0201a9fbe4cbaeb0c0a9b83f56f4af74eff95b
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>