power: pm8921-charger: don't reject all the chg current requests

The driver currently rejects any changes to the USB charge current if
no other source of power is present. This causes issues because if a
pc usb cable which supplies 500mA is connected while the PMIC was
drawing 1500mA, it will continue to draw 1500mA from it causing charger
collapse.

The reason why the rejects were put in place were because if we are
running from USB as the source, vbus_draw() could be called with 0mA.
This would cause brownouts.

So only reject setting the current to 0mA in case another source is not
present.

Change-Id: I55a829db369bf60de2ba86a522480dacddc02186
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>