net: dsa: qca8k: fix missing unlock on error in qca8k_vlan_(add|del)

Add the missing unlock before return from function qca8k_vlan_add()
and qca8k_vlan_del() in the error handling case.

Fixes: 028f5f8e ("net: dsa: qca8k: handle error with qca8k_read operation")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>