sunbmac: Remove unused local variable.

Commit eb716c54b1c71ad28ab20461bff831bd481066c4 ("sunbmac: remove
unnecessary setting of skb->dev") caused the local varible 'dev'
in bigmac_init_rings to become unused.  And now the compiler
warns about it.

Signed-off-by: David S. Miller <davem@davemloft.net>