power: qpnp-fg: fix issues with iterm configuration

For high values of termination current, the calculation from microamp
values to adc raw values can overflow, causing the fuel gauge driver to
set unrealistic values. Also, the device tree property for this
termination current was named incorrectly in the bindings documentation.

Fix these two issues in order for correct iterm configuration.

CRs-Fixed: 771380
Change-Id: Ib9be3b92e44eedeaad0fda36a79bc8f8b787ea15
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>