efi_selftest: efi_st_memcmp return difference of bytes

If the memory regions are different efi_st_memcmp currently
returns the difference of the addresses. Insted the
difference of the first differing byte pair should be
returned.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>
diff --git a/lib/efi_selftest/efi_selftest_util.c b/lib/efi_selftest/efi_selftest_util.c
index c9c295e..5cffe38 100644
--- a/lib/efi_selftest/efi_selftest_util.c
+++ b/lib/efi_selftest/efi_selftest_util.c
@@ -17,7 +17,7 @@
 
 	for (; length; --length) {
 		if (*pos1 != *pos2)
-			return pos1 - pos2;
+			return *pos1 - *pos2;
 		++pos1;
 		++pos2;
 	}