pda_power: Complain if regulator operations fail

Rather than silently ignoring errors from the regulator enable and disable
add a WARN_ON() - it's probably pretty important if we're not getting
power, though it should be vanishingly unlikely in production.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>