regulator: max8998: Use uV in voltage_map_desc

Integer division may truncate.
This happens when pdata->buckx_voltagex setting is not align with 1000 uV.
Thus use uV in voltage_map_desc, this ensures the selected voltage won't less
than pdata buckx_voltagex settings.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@vger.kernel.org