Removing ADDITIONAL_CONTROL_4 from the list of readable registers cause
audio distortion.

This change was sent as a comment below the --- line when submitting
commit 658bb297 ("ASoC: wm8962: Do not access WM8962_GPIO_BASE"), so
it was not supposed to get merged.

Keep WM8962_ADDITIONAL_CONTROL_4 inside wm8962_readable_register() to
fix the regression.

Fixes: 658bb297 ("ASoC: wm8962: Do not access WM8962_GPIO_BASE")
Reported-by: NShengjiu Wang <shengjiu.wang@gmail.com>
Signed-off-by: NFabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20200803115233.19034-1-festevam@gmail.comSigned-off-by: NMark Brown <broonie@kernel.org>