ASoC: wm8731: Drop wm8731_writeable function

When .max_register is set and .writeable_reg is not implement, registers
between 0 and .max_register are writeable. This is the same as current
implementation of wm8731_writeable(), so just drop implementation for
.writeable_reg callback.

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