[SCSI] hpsa: do readl after writel in main i/o path to ensure commands don't get lost.

Apparently we've been doin it rong for a decade, but only lately do we
run into problems.

Signed-off-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <jbottomley@parallels.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>