usb gadget: defer obex enumeration

Some USB peripheral controller drivers support software control
over the data pullup.  Use those controls to prevent the OBEX
function from enumerating until the userspace server has opened
the /dev/ttyGS* node it will use to implement protocol chitchat
with the USB host.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>