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.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>