scsi: qla2xxx: Use shadow register for ISP27XX

For ISP27XX, use shadow register to read FW provided REQQ's consumer
index. The shadow register is dma'ed by firmware.

Signed-off-by: Quinn Tran <quinn.tran@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>