power: pm8921-charger: do not open OVP on USB suspend

When the usb_suspend bit is set and an the CHG_GONE
interrupt is triggered the ovp fets are opened to
enforce a usb unplug detection.

Fix this by checking the CHG_USB_SUSPEND bit and do not
open the USB OVP FET.

Change-Id: Ibec1908c959f08fb531e43c26b61e1c455b9e6a7
Signed-off-by: David Keitel <dkeitel@codeaurora.org>