ASoC: Return -1 instead of -EINVAL to ensure consistency

The code can't really cope with I/O errors, so it would be better
to be consistent throughout all cache functions and return -1 instead
of -EINVAL.

The return value of snd_soc_read(...) is mostly checked in the probe
function and nowhere else.

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>