net: wireless: bcmdhd: Fix watchdog bark issue

Issue:
  Watch dog bark happens while we call dhd_find_sta or dhd_del_sta.
Root cause:
  Currently we don't delete the sta list when we turn off SOFTAP.
However, the each entry in sta_list is already deinitialized
in dhd_sta_pool_clear function. That means that  the sta list of
primary interface has invalid data even though we turn off the SOFTAP.

If we turn on SOFTAP again, the sta list will be reused again.

However, if sometimes the each entry of sta_list has random memory address
and point to another address for next pointer, the iterate never stop in
list_for_each_entry function.

Solution:
  We need to clean up the sta list for primary interface in dhd_stop

b/18887219

Change-Id: Icf6f3e656df36ab9816cf52cb9ddc19520d005d1
Signed-off-by: Ecco park <eccopark@broadcom.com>