ASoC: davinci-mcasp: Rename private struct and it's users (dev -> mcasp)

Rename the private struct from davinci_audio_dev to davinci_mcasp.
Change the local use of the pointer to this struct from *dev to *mcasp.

The aim is to have better readable code for the first look since having
dev->xxxx in the code when using the local private struct is a bit
surprising.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>