[SCSI] pm8001: raise host can queue

This is a followup to a patch provided by Jack Wang on September 21 2011.

After increasing the CAN_QUEUE to 510 in pm8001 we discovered some performance
degredation from time to time. We needed to increase the MPI queue to
compensate and ensure we never hit that limit. We also needed to double
the margin to support event and administrivial commands that take from
the pool resulting in an occasional largely unproductive command completion
with soft error to the caller when the command pool is overloaded temporarily.

Signed-off-by: Mark Salyzyn <mark_salyzyn@xyratex.com>
Acked-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>