net: ethernet: Use swap() instead of open coding it

Clean the following coccicheck warning:

./drivers/net/ethernet/sunplus/spl2sw_driver.c:217:27-28: WARNING
opportunity for swap().

./drivers/net/ethernet/sunplus/spl2sw_driver.c:222:27-28: WARNING
opportunity for swap().

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>