nrf5x: Fix endpoint internal state when closed

Field started (regardind transfer) was only cleaed when transfer
was finished.
For audio devices set interface is called many times.
When there is no audio (silence) set interface requests zero
lenght bandwithd that in turn calls dcd_edpt_close().

When endpoint is closed due to set interface request transfer
should not longer be started since it will block next start transfer
with assert.

This just sets 'started' to false when endpoint is closed.

src/portable/nordic/nrf5x/dcd_nrf5x.c

@@ -438,6 +438,7 @@ void dcd_edpt_close (uint8_t rhport, uint8_t ep_addr)
     // When both ISO endpoint are close there is no need for SOF any more.
     if (_dcd.xfer[EP_ISO_NUM][TUSB_DIR_IN].mps + _dcd.xfer[EP_ISO_NUM][TUSB_DIR_OUT].mps == 0) NRF_USBD->INTENCLR = USBD_INTENCLR_SOF_Msk;
   }
+  _dcd.xfer[epnum][dir].started = false;
   __ISB(); __DSB();
 }