usbcore: help drivers to change device configs

It's generally a bad idea for USB interface drivers to try to change a
device's configuration, and usbcore doesn't provide any way for them
to do it.  However in a few exceptional circumstances it can make
sense.  This patch (as767) adds a roundabout mechanism to help drivers
that may need it.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>