regulator: core: Only propagate voltage changes to if it can change voltages

When we are propagating voltage changes to parent regulators don't
bother if the parent does not have permission to change voltages.  This
simplifies error checking in the function for cases where the regulator
lacks some of the voltage operations.

Reported-by: Dong Aisheng <aisheng.dong@nxp.com>
Tested-by: Dong Aisheng <aisheng.dong@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>