ASoC: fsl: remove redundant Kconfig option SND_SOC_FSL_SSI

While commit 606d620 (ASoC: imx: merge sound/soc/imx into sound/soc/fsl)
adds SND_SOC_FSL_SSI outside "menuconfig SND_POWERPC_SOC" to make it
visible for both PowerPC and ARM/IMX, it forgot removing the one inside
"menuconfig SND_POWERPC_SOC".

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>