[SCSI] arcmsr: irq handler fixes, cleanups, micro-opts

* Remove IRQF_DISABLED, it is clearly wrong for this driver.

* Remove wasteful spin_lock_irqsave() in interrupt handler.
  The lighter-weight spin_lock() is all that's needed.

* Annotate with FIXME where arcmsr_interrupt() is called
  without any spinlock being acquired.

* Eliminate pointless cast from void pointer in arcmsr_do_interrupt()

[jejb: conflict resolution]
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Acked-by: Nick Cheng <nick.cheng@areca.com.tw>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>