usb gadget: fix fsl_usb2_udc potential OOPS

For fsl_usb2_udc driver, ep0 also has a descriptor.  Current code is
misleading and contains a logical mistake.  Here is the patch to fix it.

 http://bugzilla.kernel.org/show_bug.cgi?id=9595

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