ASoC: adau1977: Fix truncation warning on 64 bit architectures

Negating ADAU1977_BLOCK_POWER_SAI_LDO_EN creates an unsigned long constant
with all bits set which on 64 bit architectures needs to be truncated to
an unsigned int, generating a warning. Add an explicit cast since we know
this is OK.

Signed-off-by: Mark Brown <broonie@linaro.org>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>