mmc: move to a SCHED_FIFO thread

Running mmcqd as a prio 120 thread forces it to compete with standard
user processes for IO performance, especially when the system is under
severe CPU load. Move it to a SCHED_FIFO thread to reduce the impact of
load on IO performance.

bug 25392275

Change-Id: I1edfe73baa25e181367c30c1f40fee886e92b60d