power: pm8921-charger: fix PC USB charging valid status

When PC USB charger is connected the driver ends up reporting
that is is not. This is because the driver incorrectly ends up
setting the target current. A non zero target is indicative
of a usb wall charger plugged in and the driver incorrectly
reports that the pc usb cable is unplugged.

CRs-Fixed: 414741
Change-Id: Ia32e581c11954163bc8ff20a7b32432a843a17b2
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>