USB: s3c2410 gadget: ensure vbus pin in input mode during read

Some CPUs in the S3C24XX series do not support readback of the
value of a pin when the pin has been configured to an IRQ.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>