ASoC: soc-core: Support debugfs entries larger than PAGE_SIZE bytes

For some codecs with large register maps, it was not possible to dump
all registers via the codec_reg file but only up to PAGE_SIZE bytes.
This patch fixes this problem.

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>