[SCSI] pm8001: enhance IOMB process modules

We set interupt cascading count of outbound queue to get better
performance, correct some unnecessary return values and some noisy
print messages.  patch attached.

Signed-off-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: Lindar Liu <lindar_liu@usish.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>