r92su: free bss in workqueue context

This patch should fix warning which was triggered by:
WARNING: at kernel/softirq.c:160 local_bh_enable+0x48/0xa0()

This warning was triggered by cfg80211_put_bss in the
disconnect path, since it was called from an irq-context
[which is not allowed].

Reported-by: José Vázquez <ppvazquezfer@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>