ALSA: pcmtest: Don't use static storage to track per device data

ALSA: pcmtest: Don't use static storage to track per device data

While there is probably only ever a single instance of such a pcmtst
device, it's still bad style to use a static variable to store per
device data. Make use of platform_get_drvdata() and
platform_set_drvdata() which fixes a data corruption if there should be
two or more such devices (or this driver is used as a template for
another driver).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20230707075058.3402832-2-u.kleine-koenig@pengutronix.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>