powerpc/mpc5200: Bestcomm fixes to ATA support

1) ata.h has dst_pa in the wrong place (needs to match what the BestComm
   task microcode in bcom_ata_task.c expects); fix it.

2) The BestComm ATA task priority was changed to maximum in bestcomm_priv.h;
   this fixes a deadlock issue experienced with heavy DMA occurring on
   both the ATA and Ethernet BestComm tasks, e.g. when downloading a large
   file over a LAN to disk.

Signed-off-by: Tim Yamin <plasm@roo.me.uk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>