ASoC: ab8500: Remove explicit initialization of driver callbacks to NULL

Fields of structs with global storage are implicitly initialized to 0/NULL,
there is usually no need to do this explicitly.

Removing the initialization of the legacy suspend/resume callback fields
also gets the driver ready for the day when they are eventually removed.

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