port-qmi: restore driver initial MTU for mhi-net when resetting link

Kernel driver mhi-net sets a custom MTU for the netdevice: use the
same value when resetting the netdevice to its initial state.

This is important especially when rmnet is used on top of mhi-net,
since rmnet netdevices get their initial MTU from the mhi-net one.

src/mm-port-qmi.c

@@ -33,6 +33,7 @@
 
 /* as internally defined in the kernel */
 #define RMNET_MAX_PACKET_SIZE 16384
+#define MHI_NET_MTU_DEFAULT   16384
 
 G_DEFINE_TYPE (MMPortQmi, mm_port_qmi, MM_TYPE_PORT)
 
@@ -977,6 +978,7 @@ internal_reset (MMPortQmi           *self,
 {
     GTask                *task;
     InternalResetContext *ctx;
+    guint                 mtu;
 
     task = g_task_new (self, NULL, callback, user_data);
 
@@ -985,12 +987,18 @@ internal_reset (MMPortQmi           *self,
     ctx->device = g_object_ref (device);
     g_task_set_task_data (task, ctx, (GDestroyNotify) internal_reset_context_free);
 
+    /* mhi_net has a custom default MTU set by the kernel driver */
+    if (g_strcmp0 (self->priv->net_driver, "mhi_net") == 0)
+        mtu = MHI_NET_MTU_DEFAULT;
+    else
+        mtu = MM_PORT_NET_MTU_DEFAULT;
+
     /* first, bring down main interface */
     mm_obj_dbg (self, "bringing down data interface '%s'",
                 mm_port_get_device (ctx->data));
     mm_port_net_link_setup (MM_PORT_NET (ctx->data),
                             FALSE,
-                            MM_PORT_NET_MTU_DEFAULT,
+                            mtu,
                             NULL,
                             (GAsyncReadyCallback) net_link_down_ready,
                             task);