usb: gadget: qti: Don't call rmnet_disconnect from ctrl_disconnect

Though the name is confusing, the g_rmnet->disconnect() callback
should only be called in cases where the RmNet function needs to
indicate control disconnect to the host, such as when the modem
goes offline. This is because frmnet_disconnect() issues a CDC
NETWORK_CONNECTION notification message on the interrupt endpoint
to indicate the control interface has gone down. However, this
should not be called from the gqti_ctrl_disconnect() function,
which is only called when the USB bus is suspended or when the
cable is disconnected--in these cases, it is invalid to try to
to queue an interrupt packet since it will always fail.

Change-Id: Idca9a9385d7be78601227d9ef56318cb03c80c19
Signed-off-by: Jack Pham <jackp@codeaurora.org>