ALSA: echoaduio: Drop superfluous volatile modifier

The dsp_registers field of struct echoaduio has the volatile modifier,
but it's basically superfluous; the field is accessed only for the
base pointer of readl() and writel(), hence marking with __iomem alone
should suffice.  OTOH, having the volatile prefix causes a compile
warning like:
  sound/pci/echoaudio/echoaudio.c:1878:14: warning: passing argument 1 of 'iounmap' discards 'volatile' qualifier from pointer target type [-Wdiscarded-qualifiers]

So it's better to drop this superfluous modifier.

Link: https://lore.kernel.org/r/20200803143958.24324-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/echoaudio/echoaudio.c

@@ -1950,8 +1950,7 @@ static int snd_echo_create(struct snd_card *card,
 		snd_echo_free(chip);
 		return -EBUSY;
 	}
-	chip->dsp_registers = (volatile u32 __iomem *)
-		ioremap(chip->dsp_registers_phys, sz);
+	chip->dsp_registers = ioremap(chip->dsp_registers_phys, sz);
 	if (!chip->dsp_registers) {
 		dev_err(chip->card->dev, "ioremap failed\n");
 		snd_echo_free(chip);

sound/pci/echoaudio/echoaudio.h

@@ -419,7 +419,7 @@ struct echoaudio {
 	short asic_code;		/* Current ASIC code */
 	u32 comm_page_phys;			/* Physical address of the
 						 * memory seen by DSP */
-	volatile u32 __iomem *dsp_registers;	/* DSP's register base */
+	u32 __iomem *dsp_registers;		/* DSP's register base */
 	u32 active_mask;			/* Chs. active mask or
 						 * punks out */
 #ifdef CONFIG_PM_SLEEP