[SCSI] mpt2sas: staged device discovery. disable_discovery module parameter is added.

Added command line option called disable_discovery.  When enabled
on the command line, the driver will not send a port_enable when loaded
for the first time.   If port_enable is not called, then there is
no discovery of devices, as well as the sas topology.  Then later if one
desires to invoke discovery, then they will need to issue a diagnostic reset.
A diagnostic reset can be issued various ways. One of the way is throught
sysfs.

Signed-off-by: Kashyap Desai <kashyap.desai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>