stmmac: fix missing module license in the main.

This patch fixes the following warning raised
when compile:

WARNING: modpost: missing MODULE_LICENSE()
in drivers/net/ethernet/stmicro/stmmac/stmmac.o

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>