[SCSI] hpsa: add readl after writel in interrupt mask setting code

This is to ensure the board interrupts are really off when
these functions return.

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>