regulator: fixed: Constify static regulator_ops

regulator: fixed: Constify static regulator_ops

The only usage of fixed_voltage_ops and fixed_voltage_clkenabled_ops is
to assign their address the ops field in the regulator_desc struct,
which is a const pointer. Make them const to allow the compiler to put
them in read-only memory.
make them const to allow the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200913084114.8851-3-rikard.falkeborn@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>