ALSA: mpu401: Fix missing initialization of irq field

The irq field of struct snd_mpu401 is supposed to be initialized to -1.
Since it's set to zero as of now, a probing error before the irq
installation results in a kernel warning "Trying to free already-free
IRQ 0".

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=44821
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/mpu401/mpu401_uart.c

@@ -554,6 +554,7 @@ int snd_mpu401_uart_new(struct snd_card *card, int device,
 	spin_lock_init(&mpu->output_lock);
 	spin_lock_init(&mpu->timer_lock);
 	mpu->hardware = hardware;
+	mpu->irq = -1;
 	if (! (info_flags & MPU401_INFO_INTEGRATED)) {
 		int res_size = hardware == MPU401_HW_PC98II ? 4 : 2;
 		mpu->res = request_region(port, res_size, "MPU401 UART");