[SCSI] aha152x, eata, u14-34f: minor irq handler cleanups

- remove pointless casts from void*

- remove needless references to 'irq' function argument, when that
  information is already stored somewhere in a driver-private struct.

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

- remove always-false tests for dev_id==NULL

- remove always-true tests for 'irq == host_struct->irq'

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

This change's main purpose is to prepare for the patchset in
jgarzik/misc-2.6.git#irq-remove, that explores removal of the
never-used 'irq' argument in each interrupt handler.

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