ALSA: line6: Reduce superfluous spinlock in midi.c

The midi_transmit_lock is used always inside the send_urb_lock, thus
it doesn't play any role.  Let's kill it.  Also, rename
"send_urb_lock" as a more simple name "lock" since this is the only
lock for midi.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/line6/midi.h

@@ -39,15 +39,10 @@ struct snd_line6_midi {
 	*/
 	int num_active_send_urbs;
 
-	/**
-		 Spin lock to protect updates of send_urb.
-	*/
-	spinlock_t send_urb_lock;
-
 	/**
 		 Spin lock to protect MIDI buffer handling.
 	*/
-	spinlock_t midi_transmit_lock;
+	spinlock_t lock;
 
 	/**
 		 Wait queue for MIDI transmission.