regulator: fan53555: Improve vsel_mask computation

In preparation for introducing support for additional regulators which
do not use the maximum number of voltage selectors available for a given
mask, improve the mask computation formula by using fls().

Note fls() requires the bitops header, hence include it explicitly and
drop bits.h which is already pulled by bitops.h.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20230406194158.963352-7-cristian.ciocaltea@collabora.com


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