ASoC: soc-core: Fix null pointer dereference

In case the codec driver did not provide a read/write function,
codec->driver->read|write will be NULL.  Ensure that we use the one
specified in codec->read|write to avoid oopsing when we access
the debugfs entries.  This is achieved by using snd_soc_read() and
snd_soc_write().

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>