stmmac: fix and review the ptp registration.

The commit commit 7086605a ("stmmac: fix error check when init ptp")
breaks the procedure added by the
commit efee95f4

 ("ptp_clock: future-proofing drivers against PTP
subsystem becoming optional")

So this patch tries to re-import the logic added by the latest
commit above: it makes sense to have the stmmac_ptp_register
as void function and, inside the main, the stmmac_init_ptp can fails
in case of the capability cannot be supported by the HW.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre TORGUE <alexandre.torgue@st.com>
Cc: Rayagond Kokatanur <rayagond@vayavyalabs.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Nicolas Pitre <nico@linaro.org>
Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>