MINOR: server: make use of srv_is_usable() instead of checking eweight

srv_is_usable() is broader than srv_is_usable() as it not only considers
the weight but the server's state as well. Future changes will allow a
server to be in drain mode with a non-zero weight, so we should migrate
to use that function instead.
diff --git a/src/server.c b/src/server.c
index 98ac2cd..857c8aa 100644
--- a/src/server.c
+++ b/src/server.c
@@ -637,7 +637,7 @@
 	/* now propagate the status change to any LB algorithms */
 	if (px->lbprm.update_server_eweight)
 		px->lbprm.update_server_eweight(sv);
-	else if (sv->eweight) {
+	else if (srv_is_usable(sv)) {
 		if (px->lbprm.set_server_status_up)
 			px->lbprm.set_server_status_up(sv);
 	}