enic: Check firmware capability before issuing firmware commands

Check if firmware supports a particular command by first checking capability
using devcmd CMD_CAPABILITY.

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