ENGR00291400 PXP: Organize PXP task queue to be FIFO

ENGR00291400 PXP: Organize PXP task queue to be FIFO

The requested PXP tasks were handled based on channel unit. All the
tasks in one channel were handled one by one, and the tasks in another
channel only can get chance after all the tasks in previous channel
were finished. So this may allow some channel occupies PXP hardware
exclusively all the time, and other channels may never get PXP services.
So this change makes the PXP task queue to be a FIFO to avoid this kind
of unfair usage for PXP.

Signed-off-by: Fancy Fang <B47543@freescale.com>