ASoC: dapm: Adapt for debugfs API change

Back in ff9fb72b (debugfs: return error values, not NULL) the
debugfs APIs were changed to return error pointers rather than NULL
pointers on error, breaking the error checking in ASoC. Update the
code to use IS_ERR() and log the codes that are returned as part of
the error messages.

Fixes: ff9fb72b

 (debugfs: return error values, not NULL)
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>