power: smb1390: Fix taper condition for VPH configuration

For VPH configuration fcc and fcc_main are same. Reducing fcc_main from
fcc can cause the charger disable condition to always be false. Fix this
by changing the main_ilim_ua type to int and add a check for subtracting
the main_fcc_ua from fcc_ua only for VBAT configuration.

Change-Id: I63a527a6de981a42d0c879151af1346191dbb4f6
Signed-off-by: Ashish Chavan <ashichav@codeaurora.org>