USB Gadget: documentation update

This patch (as1102) clarifies two points in the USB Gadget kerneldoc:

	Request completion callbacks are always made with interrupts
	disabled;

	Device controllers may not support STALLing the status stage
	of a control transfer after the data stage is over.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>