gianfar: Fix build.

Reported by Michael Guntsche <mike@it-loops.com>

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Commit
38bddf04

 gianfar: gfar_remove needs to call unregister_netdev()

breaks the build of the gianfar driver because "dev" is undefined in
this function. To quickly test rc9 I changed this to priv->ndev but I do
not know if this is the correct one.
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Signed-off-by: David S. Miller <davem@davemloft.net>