power: pm8921-charger: OVP_FET control for USBIN drop error

In some case, USBIN voltage doesn't drop for a while when
usb cable is unplugged. Due to this error, charger cannot
be detected correctly. This patch introduce OVP_FET
off method to drop USBIN volatge in error case.

Change-Id: I8d279cc8b4e7a489e8bfdc86b2015ff45704f832