mmc: core: Enable multiple urgent requests notification

At the moment if the block layer issues an URGENT request notification
before receiving a complete on the URGENT request currently handled by
the device driver - the system crashes with a BUG_ON().
This change enables multiple block layer urgent request notification to
be handled by MMC layer by removing single notification enforcement.

Change-Id: I801f524bfa09766168027a4ab673ce9afb3d2103
CRs-fixed: 517724
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>