dsp563xx_once: fix warning and potential bug

I don't think dsp563xx_once_read_register() would ever
be called with len==0, but it would have been broken in
that case.

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

src/target/dsp563xx_once.c

@@ -188,7 +188,7 @@ int dsp563xx_once_request_debug(struct jtag_tap *tap, int reset_state)
 int dsp563xx_once_read_register(struct jtag_tap *tap, int flush, struct once_reg *regs, int len)
 {
 	int i;
-	int err;
+	int err = ERROR_OK;
 
 	for (i = 0; i < len; i++)
 	{