qla2xxx: Fix regression introduced by target configFS changes

this patch fixes following regression

 # targetcli
 [Errno 13] Permission denied: '/sys/kernel/config/target/qla2xxx/21:00:00:0e:1e:08:c7:20/tpgt_1/enable'

Fixes: 2eafd729

 ("target: use per-attribute show and store methods")
Signed-off-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: Giridhar Malavali <giridhar.malavali@qlogic.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>