staging: brcm80211: bug fix- rmmod hang problem

Bug fix - rmmod hang problem.
Can keep both of kthread & down_interruptible.
And in the meantime, can terminate the threads
properly during rmmod process.

Signed-off-by: Nohee Ko <noheek@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>