regulator: core: Limit propagation of parent voltage count and list

Commit 26988efe ("regulator: core: Allow to get voltage count and
list from parent") introduces the propagation of the parent voltage
count and list for regulators that don't provide this information
themselves. The goal is to support simple switch regulators, however as
a side effect normal continuous regulators can leak details of their
supplies and provide consumers with inconsistent information.

Limit the propagation of the voltage count and list to switch
regulators.

Fixes: 26988efe

 ("regulator: core: Allow to get voltage count and
  list from parent")
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>