regulator: max8952 - fix max8952_set_voltage

In current implementation, vid is declared as u8,
then "vid == -1" is always false, and "vid >= 0" is always true.
Thus change it to s8.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>