scsi: qla2xxx: Add mode control for each physical port

Add ability to allow each physical port to control operating mode.  Current
code forces all ports to behave in one mode (i.e. initiator, target or
dual). This patch allows user to select the operating mode for each port.

- Driver must be loaded in dual mode to allow resource allocation

modprobe qla2xxx qlini_mode=dual

- In addition user can make adjustment to exchange resources using following
  command

echo 1024 > /sys/class/scsi_host/host<x>/ql2xiniexchg
echo 1024 > /sys/class/scsi_host/host<x>/ql2xexchoffld

- trigger mode change and new setting of ql2xexchoffld|ql2xiniexchg

echo [<value>] > /sys/class/scsi_host/host<x>/qlini_mode

where, value can be one of following
  - enabled
  - disabled
  - dual
  - exclusive

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>