Allwinner H6 has a different bit to flush the TX FIFO.

Add a quirks to prepare introduction of H6 SoC.
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>