scsi: mpt3sas: Cache enclosure pages during enclosure add.

In function _scsih_add_device, for each device connected to an
enclosure, driver reads the enclosure page(To get details like enclosure
handle, enclosure logical ID, enclosure level etc.)

With this patch, instead of reading enclosure page everytime, driver
maintains a list for enclosure device(During enclosure add event,
enclosure device is added to the list and removed from the list on
delete events) and uses the enclosure page from the list.

Signed-off-by: Chaitra P B <chaitra.basappa@broadcom.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>