net: phy: mscc: fix a possible double unlock

On vsc8584_ptp_init failure we jump to the 'err' label, which unlocks
the MDIO bus lock. But vsc8584_ptp_init isn't called with the MDIO bus
lock taken, which could result in a double unlock. Fix this.

Fixes: ab2bf933

 ("net: phy: mscc: 1588 block initialization")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Antoine Tenart <antoine.tenart@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>