airo: Kill directly reference of netdev->priv

We have some reasons to kill netdev->priv:
1. netdev->priv is equal to netdev_priv().
2. netdev_priv() wraps the calculation of netdev->priv's offset, obviously
   netdev_priv() is more flexible than netdev->priv.
But we cann't kill netdev->priv, because so many drivers reference to it
directly.

OK, becasue Dave S. Miller said, "every direct netdev->priv usage is a bug",
and I want to kill netdev->priv later, I decided to convert all the direct
reference of netdev->priv first.

In this driver, I don't simply use netdev_priv() to replace netdev->priv.

The reason is:
Pointer netdev->priv was changed in this driver, but it shouldn't.
Because the memory was allocated when alloc_netdev() and netdev->priv
should always point to that memory.

So I use netdev->ml_priv to replace netdev->priv.
After replacing, both ai and ai->wifidev->ml_priv point to the same memory.
Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Cc: John W. Linville <linville@tuxdriver.com>
Cc: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>