Allow passing a void pointer to rt2x00_queue_entry_for_each which in
turn in provided to the callback function.

Furthermore, allow the callback function to stop processing by returning
true. And also notify the caller of rt2x00_queue_entry_for_each if the
loop was canceled by the callback.

No functional changes, just preparation for an upcoming patch.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>