USB: add association.h

This will be used by the wireless usb code, as well as potentially other
USB code.

Originally based on some .c code written by Inaky Perez-Gonzalez
<inaky.perez-gonzalez@intel.com>

Cc: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>