usb: musb: dsps: implement clear_ep_rxintr() callback

usb: musb: dsps: implement clear_ep_rxintr() callback

During dma teardown for dequque urb, if musb load is high, musb might
generate bogus rx ep interrupt even when the rx fifo is flushed. In such
case any of the follow log messages could happen.

    musb_host_rx 1853: BOGUS RX2 ready, csr 0000, count 0

    musb_host_rx 1936: RX3 dma busy, csr 2020

As mentioned in the current inline comment, clearing ep interrupt in the
teardown path avoids the bogus interrupt, so implement clear_ep_rxintr()
callback.

This bug seems to be existing since the initial driver for musb support,
but I only validated the fix back to v4.1, so only cc stable for v4.1+.

cc: stable@vger.kernel.org # 4.1+
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>