power: smb1390: Don't allow negative CP_ILIM override votes

After both charge pumps get disabled in taper, userspace revokes its
override request with a negative vote. This negative vote is supposed to
cause the voter to remove itself from the election by voting "false".
But this doesn't happen and the negative value gets processed by the
voter callback and gets written to the register, which manifests as a
maxing out of the ILIM value, which is not the intended behaviour.

Prevent negative values of CP ILIM requests from being set by making the
voter vote "false" for those values.

Change-Id: If8ff30ae14a4770a0a8a3f17a45b41deedfea1e0
Signed-off-by: Guru Das Srinagesh <gurus@codeaurora.org>