stmmac: fix GMAC syn ID

Erroneously the DWMAC_CORE_3_40 was set to 34 instead of 0x34.
This can generate problems when run on old chips because
the driver assumes that there are the extra 16 regs available
for perfect filtering.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Gianni Antoniazzi <gianni.antoniazzi-ext@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>