ASoC: rcar: remove unused variable

After a recent cleanup, the soc_card variable became unused
and now produces a warning:

soc/sh/rcar/core.c: In function '__rsnd_kctrl_new':
soc/sh/rcar/core.c:801:23: warning: unused variable 'soc_card' [-Wunused-variable]

This removes the variable.

Fixes: 1a497983 ("ASoC: Change the PCM runtime array to a list")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>