M263: Fix channel release in analogout_free()

Author: cyliangtw

targets/TARGET_NUVOTON/TARGET_M261/analogout_api.c

@@ -115,7 +115,7 @@ void analogout_free(dac_t *obj)
     /* Channel-level windup from here */
 
     /* Mark channel free */
-    dac_modinit_mask[modidx] &= ~(1 << modidx);
+    dac_modinit_mask[modidx] &= ~(1 << chn);
 
     /* Close channel */
     DAC_Close(dac_base, chn);