[SCSI] mac_esp: fix for quadras with two esp chips

[SCSI] mac_esp: fix for quadras with two esp chips

On the Quadra 900 and 950 there are two ESP chips sharing one IRQ. Because
the shared IRQ is edge-triggered, we must make sure that an IRQ transition
from one chip doesn't go unnoticed when the shared IRQ is already active
due to the other. This patch prevents interrupts getting lost so that both
SCSI busses may be used simultaneously.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>