ASoC: omap-mcbsp: fix snd_pcm_hw_rule_add arguments

We are setting SNDRV_PCM_HW_PARAM_BUFFER_SIZE based on
SNDRV_PCM_HW_PARAM_CHANNELS, not vice versa. This bug didn't
have much impact because the rules are evaluated multiple times
by the core, and intended value got set eventually.

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>