gianfar: reduce stack usage in gianfar_ethtool.c

  drivers/net/gianfar_ethtool.c:765: warning: the frame size of 2048 bytes is larger than 1024 bytes

Signed-off-by: Wang Shaoyan <wangshaoyan.pt@taobao.com>
Reviewed-and-tested-by: Sebastian Pöhn <sebastian.poehn@belden.com>
Signed-off-by: David S. Miller <davem@davemloft.net>