scsi: qla2xxx: Check for Register disconnect

During adapter shutdown process check for register disconnect before
proceeding to call PCI functions.

Signed-off-by: Sawan Chandak <sawan.chandak@cavium.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@cavium.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>