qla2xxx: Add missing ->vport_slock while calling qlt_update_vp_map

All other callers of qlt_update_vp_map() already hold ->vport_slock
while updating the vp target map, so go ahead and add the missing
->vport_slock within qla24xx_disable_vp() code.

Cc: Saurav Kashyap <saurav.kashyap@qlogic.com>
Cc: Chad Dupuis <chad.dupuis@qlogic.com>
Cc: Arun Easi <arun.easi@qlogic.com>
Cc: Andrew Vasquez <andrew.vasquez@qlogic.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Roland Dreier <roland@purestorage.com>
Acked-by: Saurav Kashyap <saurav.kashyap@qlogic.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>