stmmac: fix error check when init ptp

This patch fixes a problem when propagated the
failure of ptp_clock_register to open function.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre TORGUE <alexandre.torgue@st.com>
Cc: Rayagond Kokatanur <rayagond@vayavyalabs.com>
Signed-off-by: David S. Miller <davem@davemloft.net>