8139cp: fix broken suspend/resume

- check that the device is up before it is enabled again;
- the descriptor ring indexes must be set to zero before
  cp_init_hw() is issued. Add a nice comment to remember
  that skb allocation failure is still not handled.

Fixes http://bugzilla.kernel.org/show_bug.cgi?id=5681

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>