power: smb1351: pick an optimal value for current_max in parallel mode

Currently, the smb1351 operating in parallel mode just displays whatever
was requested of it as the CURRENT_MAX power supply property. However,
this does not reflect the true current limit in hardware due the the
smb1351 having a non standard current limit resolution. This can cause
the parallel charging algorithm to pick a non-optimal value for the
smb1351.

To fix this, instead of just saving whatever value the main charger sets
the current limit to, pick the closest input current limit setpoint.

CRs-Fixed: 920791
Change-Id: I17c85e0f8e557429326b2e1342ab22e2e4319eee
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>