regulator: mc13xxx: Remove mc13xxx_sw_regulator_is_enabled function

If .is_enabled callback is not implemented, regulator core assumes that the
regulator is always on. Thus we don't need mc13xxx_sw_regulator_is_enabled
function.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>