Fix comparison using wrong pointer variable in dma debug code

cppcheck reported:
[lib/dma-debug.c:248] -> [lib/dma-debug.c:248]: (style) Same expression on both sides of '=='.

Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>