Fixed regulator can't change voltage and regulator_sync_voltage()
returns -EINVAL in this case. Make regulator_sync_voltage() to succeed
for regulators that are incapable to change voltage.

On NVIDIA Tegra power management driver needs to sync voltage and we have
one device (Trimslice) that uses fixed regulator which is getting synced.
The syncing error isn't treated as fatal, but produces a noisy error
message. This patch silences that error.
Signed-off-by: NDmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20211021183308.27786-1-digetx@gmail.comSigned-off-by: NMark Brown <broonie@kernel.org>