ASoC: bcm2835: Add S16_LE support via packed DMA transfers

The bcm2835-i2s driver already has support for the S16_LE format but
that format hasn't been made available because dmaengine_pcm didn't
support packed data transfers.

bcm2835-i2s needs 16-bit left+right channel data to be packed into
a 32-bit word, the FIFO register is 32-bit only and doesn't support
16-bit access.

Now that dmaengine_pcm supports packed transfers the format can
be made available by setting the SND_DMAENGINE_PCM_DAI_FLAG_PACK flag.

No further configuration is necessary:
- snd_dmaengine_dai_dma_data.addr_width is already set to
  DMA_SLAVE_BUSWIDTH_4_BYTES to force 32-bit DMA transfers
- dmaengine_pcm will pick up the S16_LE format from the DAI
  configuration and make it available since it's no longer
  masked out due to the PACK flag.
- there are no further corner cases to catch in hw_params,
  since the channel count is fixed at 2 we always have two
  16-bit stereo samples that can be transferred via 32-bit DMA

Signed-off-by: Matthias Reichl <hias@horus.com>
Tested-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>