staging: brcm80211: fix rtnl_lock issue when bringing down brcmfmac

When bringing down the netdevice interface a deadlock occurred
sporadically due to the rtnl_lock being held by a task that was
waiting for another task trying to get the lock. This patch fixes
that issue.

Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>