regulator: cros-ec: Constify cros_ec_regulator_voltage_ops

It is never modified, so make it const to allow the compiler to put it
in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Pi-Hsun Shih <pihsun@chromium.org>
Link: https://lore.kernel.org/r/20200711114409.9911-1-rikard.falkeborn@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>