[SCSI] mpt2sas: Fix for staged device discovery functionality of driver not working

This patch provides a command line option to disable "Port enable" during
the driver load.

The objective of this command line option is to load the driver and do
all the necessary initialization excluding port enable(i.e. delay
device discovery)

Signed-off-by: Sreekanth Reddy <sreekanth.reddy@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>