usb: musb: fix tx fifo flush handling again

commit 68fe05e2a4 ("usb: musb: fix tx fifo flush handling") drops the
1ms delay trying to solve the long disconnect time issue when
application queued many tx urbs. However, the 1ms delay is needed for
some use cases, for example, without the delay, reconnecting AR9271 WIFI
dongle no longer works if the connection is dropped from the AP.

So let's add back the 1ms delay in musb_h_tx_flush_fifo(), and solve the
long disconnect time problem with a separate patch for
usb_hcd_flush_endpoint().

Cc: stable@vger.kernel.org # v4.4+
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/musb_host.c

@@ -139,6 +139,7 @@ static void musb_h_tx_flush_fifo(struct musb_hw_ep *ep)
 				"Could not flush host TX%d fifo: csr: %04x\n",
 				ep->epnum, csr))
 			return;
+		mdelay(1);
 	}
 }