move virtnet_send_command() above virtnet_set_mac_address()

We want to send vq command to set mac address in
virtnet_set_mac_address(), so do this function moving.
Fixed a little issue of coding style.

Signed-off-by: Amos Kong <akong@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>