ieee802154: hwsim: fix missing unlock on error in hwsim_add_one()

Add the missing unlock before return from function hwsim_add_one()
in the error handling case.

Fixes: f25da51f ("ieee802154: hwsim: add replacement for fakelb")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>