power: qpnp-smbcharger: allow lowering of charger current

Once parallel charger is activated, the next AICL could result in
lower charge current than the initial one. Allow for lowering
the charger current, since enabling charging via two sources
reduces thermal losses.

At the same time restrict the lowering of current to prevent
any unreasonable charger current drops.

CRs-Fixed: 733213
Change-Id: I70cd64b67112bb26e9afd5b9d1b04ed9a407ba8e
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>