ASoC: mediatek: mt6359: fix kselftest error of playback gain

ASoC: mediatek: mt6359: fix kselftest error of playback gain

kselftest tries to read/write the default value. The default register
value of playback gain is 0x1F(mute), but max gain we specified is 0x12.
The range of the control is 0x0~0x12 and mute(0x1F) is only used in the
driver internally. To solve the problem, implement a new callback
mt6359_get_playback_volsw to report user configured volume instead of
the register value.

In addition, update max of "Headset Volume" to 0x12, so it can match the
maximum seen on latest data sheet.

Signed-off-by: Trevor Wu <trevor.wu@mediatek.com>
Link: https://lore.kernel.org/r/20230508071532.21665-3-trevor.wu@mediatek.com
Signed-off-by: Mark Brown <broonie@kernel.org>