[SCSI] qla2xxx: Reduce lock-contention during do-work processing.

Queued work processing will now be serialized with its own
lower-priority spinlock.  This also simplifies the work-queue
interface for future work-queue consumers.

Signed-off-by: Andrew Vasquez <andrew.vasquez@qlogic.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>