net/dhcp: Make grub_net_process_dhcp() take an interface

Change the interface of the function dealing with incoming BOOTP packets
to take an interface instead of a card, to allow more fine per-interface
state (timeout, handshake state) later on.

Use the opportunity to clean up the code a bit.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/net.h

@@ -475,7 +475,7 @@ grub_net_add_ipv4_local (struct grub_net_network_level_interface *inf,
 
 void
 grub_net_process_dhcp (struct grub_net_buff *nb,
-		       struct grub_net_card *card);
+		       struct grub_net_network_level_interface *iface);
 
 int
 grub_net_hwaddr_cmp (const grub_net_link_level_address_t *a,