usb: f_rmnet: Replace fifo_flush with ep_dequeue for notify_req

Some UDC drivers e.g. dwc3 don't implement fifo_flush callback.
Driver currently uses this to dequeue notify_req. Since, there
is only one notify_req it is safe to replace to fifo_flush with
ep_dequeue. Otherwise drivers ends up purging responses on suspend
when remote-wakeup is disabled but doesn't dequeue notify_request.
Later when host tries to fetch response after resume device returns
STALL and host reports error with adapter.

Change-Id: Ib0755690af1fd150a30d8e9085f7384b0e403add
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>