mmc: queue: Share mmc request array between partitions
eMMC can have multiple internal partitions that are represented as separate
disks / queues. However switching between partitions is only done when the
queue is empty. Consequently the array of mmc requests that are queued can
be shared between partitions saving memory.
Keep a pointer to the mmc request queue on the card, and use that instead
of allocating a new one for each partition.