ASoC: pxa-ssp: drop SNDRV_PCM_FMTBIT_S24_LE

This mode is unsupported, as the DMA controller can't do zero-padding
of samples.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org