ASoC: tpa6130a2: Fix unbalanced regulator disables

This driver has unbalanced regulator_disable when doing module loading and
unloading. This is because tpa6130a2_probe followed by tpa6130a2_remove
calls twice tpa6130a2_power(0). Fix this by implementing a state checking
in tpa6130a2_power.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Cc: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>