mvp: fixed default device detection

The check "open()!=0" is wrong, you have to write "open()>=0", because
-1 means error, and 0 is a valid file handle.

NEWS

@@ -20,6 +20,7 @@ ver 0.15 - (200?/??/??)
   - mvp: fall back to stereo
   - mvp: fall back to 16 bit audio samples
   - mvp: check for reopen errors
+  - mvp: fixed default device detection
 * commands:
   - "playlistinfo" supports a range now
   - added "sticker database", command "sticker", which allows clients

src/output/mvp_plugin.c

@@ -114,7 +114,7 @@ mvp_output_test_default_device(void)
 
 	fd = open("/dev/adec_pcm", O_WRONLY);
 
-	if (fd) {
+	if (fd >= 0) {
 		close(fd);
 		return true;
 	}