drivers/net: enic: Add an interface for USNIC to interact with firmware

This patch adds an interface for USNIC to proxy firmware commands
through ENIC.

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