ASoC: dapm: add initialization for w_param_text pointer

We now allocate the array conditionally, but we always pass
the pointer to the new snd_soc_dapm_free_kcontrol() function,
which introduces a warning for the case that it is not
initialized:

sound/soc/soc-dapm.c: In function 'snd_soc_dapm_new_pcm':
sound/soc/soc-dapm.c:3940:2: error: 'w_param_text' may be used uninitialized in this function [-Werror=maybe-uninitialized]

As snd_soc_dapm_free_kcontrol() is global, it doesn't get inlined
and gcc fails to notice that we don't actually access the array
in that case, so the code is actually safe. Adding an initialization
for the array pointer shuts up the warning.

Fixes: 19ad683a ("ASoC: dapm: Avoid creating kcontrol for params")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>