[SCSI] aha1542: minor irq handler cleanups

- where the 'irq' function argument is known never to be used, rename
  it to 'dummy' to make this more obvious

- replace per-irq lookup functions and tables with a direct reference
  to data object obtained via 'dev_id' function argument, passed from
  request_irq()

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>