ASoC: rt5645: Rename HP control to Headphone

Use the standard name "Headphone" instead of "HP" for better
userspace integration.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Acked-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>