ASoC: wm8741: Drop misleading readable_reg callback implementation

regmap_readable() returns false if map->format.format_write is set.
For .reg_bits = 7, .val_bits = 9, setting,
  map->format.format_write = regmap_format_7_9_write;

Even current code has implemented map->readable_reg, regmap_readable()
still returns false anyway. Thus drop the misleading readable_reg callback
implementation.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>