net: lwip: move eth_init() out of new_netif()

Move the initialization of the ethernet devices out of the new_netif()
function. Indeed, new_netif() accepts a struct device argument, which
is expected to be valid and active. The activation and selection of
this device are achieved by eth_init() (on first time the network
stack is used) and eth_set_current(). This is what takes care of the
ethrotate and ethact environment variables. Therefore, move these calls
to a new function: net_lwip_set_current(), and use it whenever a
net-lwip command is run.

This patch hopefully fixes the incorrect net-lwip behavior observed on
boards with multiple ethernet interfaces [1].

Tested on an i.MX8MPlus EVK equipped wih two ethernet ports. The dhcp
command succeeds whether the cable is plugged into the first or second
port.

[1] https://lists.denx.de/pipermail/u-boot/2025-January/576326.html

Reported-by: E Shattow <e@freeshell.de>
Tested-by: E Shattow <e@freeshell.de>
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>

net/lwip/dns.c

@@ -121,7 +121,7 @@ int do_dns(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 	if (argc == 3)
 		var = argv[2];
 
-	eth_set_current();
+	net_lwip_set_current();
 
 	return dns_loop(eth_get_dev(), name, var);
 }