libertas: clean up association debug messages

This makes the debug output of all association stuff clearer by:

* adding some lbs_deb_enter()/lbs_deb_leave() calls
* printing the return level in one place
* lower-casing some string

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>