ASoC: dwc: Unmask I2S interrupts only for enabled channels

There is no need to unmask all interrupts at I2S start. This
can cause performance issues in slower platforms.

Unmask only the interrupts for the used channels.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: Mark Brown <broonie@kernel.org>