[ARM] tegra: tegra_i2s_audio: allow preloading of the tx fifo with data

[ARM] tegra: tegra_i2s_audio: allow preloading of the tx fifo with data

Add an ioctl to allow the TX fifo to be loaded with data before playback
starts.  Playback can then be started by calling write() on the FIFO, even
with a length of 0.  This will cause the pending data to be played out.

Signed-off-by: Iliyan Malchev <malchev@google.com>