ASoC: pcm1681: Fix max_register setting

According to the datasheet, the max_register is 13h.
ARRAY_SIZE(pcm1681_reg_defaults) + 1 is 18 which is wrong.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>