regulator: pf8x00: Use regulator_map_voltage_ascend for pf8x00_buck7_ops

The voltages in pf8x00_sw7_voltages are in ascendant order, so use
regulator_map_voltage_ascend.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Adrien Grassein <adrien.grassein@gmail.com>
Link: https://lore.kernel.org/r/20210216060128.126938-1-axel.lin@ingics.com
Signed-off-by: Mark Brown <broonie@kernel.org>