sfc: Use swap() instead of open coding it

Clean the following coccicheck warning:

./drivers/net/ethernet/sfc/efx_channels.c:870:36-37: WARNING opportunity
for swap().

./drivers/net/ethernet/sfc/efx_channels.c:824:36-37: WARNING opportunity
for swap().

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