staging-p80211: Kill directly reference of netdev->priv

In this driver, netdev's private data is wlandevice_t.  And the
wlandev(type of wlandevice_t) is exist before netdev be allocated.  So
use netdev->ml_priv to point to the private data.

I am not sure whether I should consider the kernel version older than
2.3.38.  Because in those kernels, netdevice_t is "structure dev"
instead of "structure net_device" and of course "dev->ml_priv" will
cause compile error.  But before my patch, in function wlan_setup(),
there is a ether_setup(net_device) which already broke kernels which
older than 2.3.38.

Signed-off-by: Wang Chen <wangchen@cn.fujitsu.com>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>