drivers/net: enic: Move ethtool code to a separate file

This patch moves all enic ethtool hooks from enic_main.c to a new file
enic_ethtool.c

Signed-off-by: Neel Patel <neepatel@cisco.com>
Signed-off-by: Christian Benvenuti <benve@cisco.com>
Signed-off-by: Nishank Trivedi <nistrive@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>