ASoC: kirkwood-i2s: fix mute handling

The spec requires that the mute bits must be set while the channel
is disabled.  Ensure that this is the case by providing a helper
which ensures that the appropriate mute bit is set while the enable
bit is clear.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Mark Brown <broonie@linaro.org>