ASoC: mop500_ab8500: Staticize non exported functions

The mop500_ab8500_startup(), the mop500_ab8500_shutdown() and the
mop500_ab8500_hw_params() function are not used outside of mop500_ab8500, so
make them static.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>