If we calculate the voltage in the table right, we get all kinds of values,
which never fit the hardware steps, so we use the closest higher value the
hardware can do.

v3: Simplify the implementation.
v5: Initialize best_err with volt->max_uv.
Signed-off-by: NKarol Herbst <karolherbst@gmail.com>
Reviewed-by: NMartin Peres <martin.peres@free.fr>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>