ASoC: tegra: allocate AHUB FIFO during probe() not startup()

The Tegra30 I2S driver currently allocates DMA FIFOs from the AHUB only
when an audio stream starts playback. This is theoretically nice for
resource sharing, but makes no practical difference for any configuration
the drivers currently support. However, this deferral prevents conversion
to the standard DMA DT bindings, since conversion requires knowledge of
the specific DMA channel to be allocated, which in turn depends on which
specific FIFO was allocated.

For this reason, move the FIFO allocation into probe() to allow later
conversion to the standard DMA DT bindings.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Brown <broonie@linaro.org>