power: smb1390: Fix low vote issue with FCC taper stepper

While in taper mode, the effective result of FCC is obtained and a
reduction of 100mA is made until the taper condition goes away. if
another voter cuts in and votes lower, which results in lower FCC, a
100mA will be deducted from the new low FCC. Note the result cannot
be reversed even after the vote is cancelled or updated.

Occasionally, it may have undesired consequences. A very low vote
voting in and out in the middle can leave a low FCC for the rest
of charging cycle.

Solve the problem by adopting a dedicated FCC voter in taper mode and
doing the reduction with reference to its last vote.

CRs-Fixed: 2357939
Change-Id: Ib8d35b21309e7cad0d3ae3dd81b491322bc37b51
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>
Signed-off-by: Harry Yang <harryy@codeaurora.org>