ASoC: blackfin: bf5xx-i2s-pcm: Use snd_pcm_lib_preallocate_pages_for_all()

Use snd_pcm_lib_preallocate_pages_for_all() for pre-allocating the DMA buffers
instead of re-implementing the same functionality.  Note that there is no need
to call snd_pcm_lib_free_pages_for_all() since the ALSA core takes care of this
for us.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>