power: pm8921-charger: reject vbus calls after checking vbat

Only reject VBUS requests after also checking the VBAT line
to ensure there is actually not other power source present.

Change-Id: Ie846e1d0450e62cddb69bd64a654214215fe2b2b
Signed-off-by: David Keitel <dkeitel@codeaurora.org>