ENGR00290664 PXP: allocate DMA TX descriptors on demand instead of in PXP initialization

ENGR00290664 PXP: allocate DMA TX descriptors on demand instead of in PXP initialization

In previous PXP driver, the number of tx descriptors allocated
for each channel is a constant 16 and they can only be allocated
during PXP initialization. But since the driver allows users to
queue more than one PXP tasks for each channel before issuing
pending tasks, so in this case the descriptors may be not enough
for some cases.

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