mac80211: set assoc flag to bss_conf

Only BSS_CHANGED_ASSOC was set in the 'changed' bitmask. Assignment  to
bss_conf.assoc was absent.
This patch assign value to  bss_conf.assoc according the association state.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>