net: dsa: sja1105: Fix missing unlock on error in sk_buff()

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

Fixes: f3097be2 ("net: dsa: sja1105: Add a state machine for RX timestamping")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>