net: stmmac: handle endianness in dwmac4_get_timestamp

GMAC IP is little-endian and used on several kind of CPU (big or little
endian). Main callbacks functions of the stmmac drivers take care about
it. It was not the case for dwmac4_get_timestamp function.

Fixes: ba1ffd74 ("stmmac: fix PTP support for GMAC4")
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>