b43: rename b43_wldev's field with ssb_device to sdev

We free name "dev" for something generic (like dev abstraction layer).
Additionaly code is cleaner now, especially magic dev->dev-dev chains.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>