scsi: qla4xxx: fix a potential NULL pointer dereference

In case iscsi_lookup_endpoint fails, the fix returns -EINVAL to avoid NULL
pointer dereference.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Acked-by: Manish Rangankar <mrangankar@marvell.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>