ALSA: add/change some comments describing function return values

script/kernel-doc reports the following type of warnings (when run in verbose
mode):

Warning(sound/core/init.c:152): No description found for return value of
'snd_card_create'

To fix that:
- add missing descriptions of function return values
- use "Return:" sections to describe those return values

Along the way:
- complete some descriptions
- fix some typos

Signed-off-by: Yacine Belkadi <yacine.belkadi.1@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/mpu401/mpu401_uart.c

@@ -129,6 +129,8 @@ static void _snd_mpu401_uart_interrupt(struct snd_mpu401 *mpu)
  * @dev_id: mpu401 instance
  *
  * Processes the interrupt for MPU401-UART i/o.
+ *
+ * Return: %IRQ_HANDLED if the interrupt was handled. %IRQ_NONE otherwise.
  */
 irqreturn_t snd_mpu401_uart_interrupt(int irq, void *dev_id)
 {
@@ -148,6 +150,8 @@ EXPORT_SYMBOL(snd_mpu401_uart_interrupt);
  * @dev_id: mpu401 instance
  *
  * Processes the interrupt for MPU401-UART output.
+ *
+ * Return: %IRQ_HANDLED if the interrupt was handled. %IRQ_NONE otherwise.
  */
 irqreturn_t snd_mpu401_uart_interrupt_tx(int irq, void *dev_id)
 {
@@ -519,7 +523,7 @@ static void snd_mpu401_uart_free(struct snd_rawmidi *rmidi)
  * not the mpu401 instance itself.  To access to the mpu401 instance,
  * cast from rawmidi->private_data (with struct snd_mpu401 magic-cast).
  *
- * Returns zero if successful, or a negative error code.
+ * Return: Zero if successful, or a negative error code.
  */
 int snd_mpu401_uart_new(struct snd_card *card, int device,
 			unsigned short hardware,