Add the missing unlock before return from error path.

Fixes: 4363f3d3 ("crypto: octeontx2 - add synchronization between mailbox accesses")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>