target: fix retval gaffe in mwX commands

failure to write to memory was not propagated.

This is an interesting case of broken error handling:
with exceptions we wouldn't have had this at all,
and I also wonder if there is a GCC option to warn
about these kinds of potential bugs.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

src/target/target.c

@@ -2378,7 +2378,7 @@ static int target_fill_mem(struct target *target,
 		{
 			current = chunk_size;
 		}
-		int retval = fn(target, address + x * data_size, data_size, current, target_buf);
+		retval = fn(target, address + x * data_size, data_size, current, target_buf);
 		if (retval != ERROR_OK)
 		{
 			break;