power: pm8921-charger: Do not reset USB input current limit

When booting without a battery there is a possiblity of browning
out a device if the input current limit of USB is reset during
the device probe.

Fix this by removing the corresponding code and let the USB OTG
driver reset the current limit when initalizing. Also add a check
for the vbus_draw API to reject any settings from the OTG driver
when there is no battery present and no other available power
source but VBUS.

Change-Id: I3f275f69ce8dfe14c35562cb75e1043ac769ce07
Signed-off-by: David Keitel <dkeitel@codeaurora.org>