regulator: s2mps11: refactor S2MPG10 ::set_voltage_time() for S2MPG11 reuse

regulator: s2mps11: refactor S2MPG10 ::set_voltage_time() for S2MPG11 reuse

The upcoming S2MPG11 support needs a similar, but different version of
::set_voltage_time(). For S2MPG10, the downwards and upwards ramps for
a rail are at different offsets at the same bit positions, while for
S2MPG11 the ramps are at the same offset at different bit positions.

Refactor the existing version slightly to allow reuse.

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://patch.msgid.link/20260122-s2mpg1x-regulators-v7-17-3b1f9831fffd@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>