ASoC: tegra: fix I2S bit count mask

This register field is 11 bits wide, not 15 bits wide. Given the way
this value is currently, used, this patch has no practical effect.
However, it's still best if the value is correct.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>