selftests/dma:remove unused variable

The variable are never referenced in the code, just remove it
that this problem was discovered by reading code

Signed-off-by: Zhu Jun <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/dma/dma_map_benchmark.c

···
 	int granule = 1;
 
 	int cmd = DMA_MAP_BENCHMARK;
-	char *p;
 
 	while ((opt = getopt(argc, argv, "t:s:n:b:d:x:g:")) != -1) {
 		switch (opt) {