staging: r8188eu: use swap()

Use swap() in order to make code cleaner. Issue found by coccinelle.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Changcheng Deng <deng.changcheng@zte.com.cn>
Link: https://lore.kernel.org/r/20210917100238.232576-1-deng.changcheng@zte.com.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>