ASoC : cs4265 : readable register too low

The cs4265_readable_register function stopped short of the maximum
register.

An example bug is taken from :
https://github.com/Audio-Injector/Ultra/issues/25

Where alsactl store fails with :
Cannot read control '2,0,0,C Data Buffer,0': Input/output error

This patch fixes the bug by setting the cs4265 to have readable
registers up to the maximum hardware register CS4265_MAX_REGISTER.

Signed-off-by: Matt Flax <flatmax@flatmax.org>
Reviewed-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Mark Brown <broonie@kernel.org>