[SCSI] mptbase: mpt_interrupt should return IRQ_NONE

The way mpt_interrupt() was coded, it was impossible for the unhandled
interrupt detection logic to ever trigger. All interrupt handlers should
return IRQ_NONE when they have nothing to do.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andrew Morton <akpm@osdl.com>
Signed-off-by: Eric Moore <Eric.Moore@lsil.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>