power: pm8921-charger: fix decrease_usb_value for PM8917

When decreasing the usb value from the usb_ma_table make
sure to take into account wether the value is available
on the PMIC platform by checking the first bit and if
the iusbfineres flag is set. This ensures that both
PM8917 and PM8921 are supported correctly.

Change-Id: I0be94103a066554924f8b212d3751f13812c49e0
Signed-off-by: David Keitel <dkeitel@codeaurora.org>