ASoC: Remove locking in snd_soc_{new,free}_ac97_codec()

snd_soc_new_ac97_codec() and snd_soc_free_ac97_codec() are called from
within a CODEC's probe() and remove() callbacks. Those will not run
concurrently against each other for the same CODEC instance, hence it is not
necessary to protect the two functions with a mutex.

This removes the last user in the ASoC core of the snd_soc_codec mutex field
and will allow us to eventually remove it.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>