Make stop on fifo threshold configurable

[BertLammers]

I would like to set a fifo watermark threshold, but still be able to use the full fifo buffer. However the current driver implementation does not support this as it always set fifo_ctrl.stop_on_fth = 0 if a fifo watermark is set.

Could this be changed, so the fifo_ctrl.stop_on_fth setting could be set independently from the fifo watermark? I can also make the change myself and provide a pull request if required.