bearer-mbim: increase disconnect timeout

It's been observed that modems may take a long time to disconnect from
the network under certain network conditions. This patch increases the
timeout for the MBIM_CID_CONNECT set command in the disconnect sequence
from 10s to 30s.

src/mm-bearer-mbim.c

@@ -1054,7 +1054,7 @@ disconnect_context_step (DisconnectContext *ctx)
 
         mbim_device_command (ctx->device,
                              message,
-                             10,
+                             30,
                              NULL,
                              (GAsyncReadyCallback)disconnect_set_ready,
                              ctx);