libertas: move association code from scan.c into assoc.c

Besides code moving, I did the following changes:

* made some functions static
* removed some unneeded #include's
* made patch checkpatch.pl clean

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