net: stmmac: Switch stmmac_hwtimestamp to generic HW Interface Helpers

Switch stmmac_hwtimestamp to generic Hardware Interface Helpers instead
of using hard-coded callbacks. This makes the code more readable and
more flexible.

No functional change.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>