ASoC: rsrc-card: remove unused ret

'04700027("ASoC: rsrc-card: cleanup for DPCM")'
cleanuped rsrc-card driver, but then, unused ret was left.
Below warning happen without this patch

${LINUX}/sound/soc/sh/rcar/rsrc-card.c: In function 'rsrc_card_startup':
${LINUX}/sound/soc/sh/rcar/rsrc-card.c:78:6: warning: unused variable \
	'ret' [-Wunused-variable]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>