ASoC: rt715: use dev_to_sdw_dev() instead of to_sdw_slave_device()

ASoC: rt715: use dev_to_sdw_dev() instead of to_sdw_slave_device()

This patch fixup this error

  CC      sound/soc/codecs/rt715-sdw.o
linux/sound/soc/codecs/rt715-sdw.c: In function 'rt715_dev_resume':
linux/sound/soc/codecs/rt715-sdw.c:568:28: error: implicit declaration\
 of function 'to_sdw_slave_device'; did you mean 'sdw_slave_modalias'?\
[-Werror=implicit-function-declaration]
  struct sdw_slave *slave = to_sdw_slave_device(dev);
                            ^~~~~~~~~~~~~~~~~~~
                            sdw_slave_modalias
linux/sound/soc/codecs/rt715-sdw.c:568:28: warning: initialization of\
 'struct sdw_slave *' from 'int' makes pointer from integer without a\
 cast [-Wint-conversion]
cc1: some warnings being treated as errors

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/87h80yhm9p.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>