net: ethernet: sun: use min_t() to make code cleaner

Use min_t() in order to make code cleaner.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>