ASoC: soc-core: Ensure codec_reg has fixed length fields

Make the format of the codec_reg file more easily parsable.  Remove
the header field which gives the codec name.  These changes are important
when it comes to extend the debugfs codec_reg file to dump more than
PAGE_SIZE bytes to make it easier to calculate offsets within the
file.

We still need to handle the case when the snd_soc_read() call fails
and <no data: %d> is outputted.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>