wl12xx: set do_join on BSS_CHANGED_ASSOC

wl12xx sets the do_join flag (which later starts the
sta role) when the bssid was changed and the sta is
associated. However, this no longer happens after
the "mac80211: remove spurious BSSID change flag"
patch.

Fix it by setting the do_join flag on BSS_CHANGED_ASSOC
(for IBSS, do_join is already set on BSS_CHANGED_IBSS)

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>