stmmac: fix an error code in stmmac_ptp_register()

PTR_ERR(NULL) is success.  We have to preserve the error code earlier.

Fixes: 7086605a ("stmmac: fix error check when init ptp")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>