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>

drivers/net/ethernet/stmicro/stmmac/stmmac_main.c

@@ -2039,3 +2039,7 @@ static int __init stmmac_cmdline_opt(char *str)
 
 __setup("stmmaceth=", stmmac_cmdline_opt);
 #endif
+
+MODULE_DESCRIPTION("STMMAC 10/100/1000 Ethernet device driver");
+MODULE_AUTHOR("Giuseppe Cavallaro <peppe.cavallaro@st.com>");
+MODULE_LICENSE("GPL");