ASoC: stm32: remove redundant pointers 'priv' and 'rtd'

Pointer 'priv' is assigned and not used, removing this allows
the removal of pointer 'rtd'.

Cleans up clang warning:
warning: variable 'priv' set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>