[ALSA] use unsigned 1-bit fields

Virtual Midi
Can't have a boolean and a sign bit in 1 bit.

Fix (14) boolean/bitfield sparse warning:
include/sound/seq_virmidi.h:41:16: warning: dubious one-bit signed bitfield
Signed-off-by: Randy Dunlap <rddunlap@osdl.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

include/sound/seq_virmidi.h

@@ -38,7 +38,7 @@ typedef struct _snd_virmidi {
 	int seq_mode;
 	int client;
 	int port;
-	int trigger: 1;
+	unsigned int trigger: 1;
 	snd_midi_event_t *parser;
 	snd_seq_event_t event;
 	snd_virmidi_dev_t *rdev;