[flash_ctrl] Fix host / controller arbitration
[flash_ctrl] Fix host / controller arbitration

- Due to the way arbitration count is cleared, it is possible
  that when controller wins the arbitration its flash transaction
  is not yet accepted.  This gives the host an opportunity to come
  back and re-win the arbitration.  This is not correct behavior.
- Clear arbitration count only when controller has won and completed the
  the transaction.

Signed-off-by: Timothy Chen <d7b2ae30d8c0f215facd721941273067fd1d14a2@google.com>

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