[SCSI] qla2xxx: Cleanup any outstanding SRB resources during shutdown.

Refactor SRB-failure completion codes in the process.  Also,
signal the DPC routine to complete sooner as backend processing
at shutdown-time is superflous.

[jejb: resolve conflicts with pci_enable_device_bars removal]

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