Isolate spinlock for tx and rx to resolve double-locking.

This is potential bug while this controller does not exist on any
SMP platforms, but lockdep or rt-preempt reveals this bug.
Reported-by: Ralf Roesch <ralf.roesch@rw-gmbh.de>
Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>