ASoC: DaVinci: i2s, remove MOD_REG_BIT macro

No functional changes. Rename variable w to something
more meaningful. Remove code obfuscating macro MOD_REG_BIT.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>