r8169: bh locking redux and task scheduling.

- atomic bit operations are globally visible
- pending status is always cleared before execution
- scheduled works are either idempotent or only required to happen once
  after a series of originating events, say link events for instance

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Suggested-by: Michał Mirosław <mirqus@gmail.com>
Cc: Hayes Wang <hayeswang@realtek.com>