video: fbmem: use swap() to make code cleaner in fb_rotate_logo()

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>
Signed-off-by: Helge Deller <deller@gmx.de>