ASoC: rt5645: make RT5650_TDM_CTRL_4 readable

Register RT5650_TDM_CTRL_4(0x7A) is readable and used for mixer
setting. It should be added in rt5645_readable_register function.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Oder Chiou <oder_chiou@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>