mpt2sas: Fix for With post diag reset same set of device gets added, removed and...

mpt2sas: Fix for With post diag reset same set of device gets added, removed and then again gets added with new target ids

When device discovery is disabled during driver load time using module
parameter "disable_discovery=1" and when diag reset is issued then from logs,
it is observed that the devices get added, removed and then added with new
target ids.

So, in order to limit this turn-off the code which is deleting and devices
across host reset when the disable_discovery module parameter is turned on.

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