scsi: mac_esp: Replace bogus memory barrier with spinlock

Commit da244654 ("[SCSI] mac_esp: fix for quadras with two esp
chips") added mac_scsi_esp_intr() to handle the IRQ lines from a pair of
on-board ESP chips (a normal shared IRQ did not work).

Proper mutual exclusion was missing from that patch. This patch fixes
race conditions between comparison and assignment of esp_chips[]
pointers.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>