[SCSI] qla2xxx: Enable queue-full throttling when UNDERRUN detected.

As ISP24xx firmware can return a CS_DATA_UNDERRUN completion
status when the storage has returned a
SAM_STAT_TASK_SET_FULL scsi-status.

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