usb: gadget: Do not pullup D+ by default on bootup

UDC core is starting device controllers as soon as gadget driver
is registered with the UDC, resulting in controller pulling up D+.
This can cause problems with drivers which registers with UDC during
initialization but are ready for enumeration only after user enables
it later. One example of such a driver is android.c

Change-Id: I8985839bd2659f3e3962fe85bd261cd63f43a991
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>