ASoC: fsl: Fix build problem

Commit 43248122 (ASoC: fsl-ssi: Use regmap) removed struct ccsr_ssi.
Unfortunately, the structure is still used. This causes
mpc85xx_smp_defconfig and mpc85xx_defconfig builds to fail with

sound/soc/fsl/fsl_dma.c:926:50:
  error: invalid use of undefined type 'struct ccsr_ssi'
  dma->ssi_stx_phys = res.start + offsetof(struct ccsr_ssi, stx0);
ound/soc/fsl/fsl_dma.c:927:50:
  error: invalid use of undefined type 'struct ccsr_ssi'
  dma->ssi_srx_phys = res.start + offsetof(struct ccsr_ssi, srx0);

Fix by using constants, similar to original commit.

Cc: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mark Brown <broonie@linaro.org>