[PATCH] zd1211rw: Fix of signal strength and quality measurement

Caused by a documentation issue I mixed up fields of the zd_status
structure. This patch fixes it and improves also the average
computation, which is now using only measurements of packets sent
by the access point.

Signed-off-by: Ulrich Kunitz <kune@deine-taler.de>
Signed-off-by: Daniel Drake <dsd@gentoo.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>