base-files: always set proto passed to _ucidef_set_interface()

Overwrite an already set proto if a new one is passed to
_ucidef_set_interface() similar to what is done for the interface.

It is required when using ""ucidef_set_interface_wan 'ptm0' 'pppoe'"
after some initial wan interface configuration is already done by
ucidef_add_switch.

The "json_is_a protocol string" guard is meant to not reset an earlier
set interface proto in case something like
"ucidef_set_interface_lan 'eth0'" is used afterwards.

Signed-off-by: Mathias Kresin <dev@kresin.me>

package/base-files/files/lib/functions/uci-defaults.sh

@@ -35,7 +35,7 @@ _ucidef_set_interface() {
 	json_select_object "$name"
 	json_add_string ifname "$iface"
 
-	if ! json_is_a protocol string; then
+	if ! json_is_a protocol string || [ -n "$proto" ]; then
 		case "$proto" in
 			static|dhcp|none|pppoe) : ;;
 			*)