ALSA: echoaudio: add a check for ioremap_nocache

In case ioremap_nocache fails, the fix releases chip and returns
an error code upstream to avoid NULL pointer dereference.
Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/echoaudio/echoaudio.c

@@ -1952,6 +1952,11 @@ static int snd_echo_create(struct snd_card *card,
 	}
 	chip->dsp_registers = (volatile u32 __iomem *)
 		ioremap_nocache(chip->dsp_registers_phys, sz);
+	if (!chip->dsp_registers) {
+		dev_err(chip->card->dev, "ioremap failed\n");
+		snd_echo_free(chip);
+		return -ENOMEM;
+	}
 
 	if (request_irq(pci->irq, snd_echo_interrupt, IRQF_SHARED,
 			KBUILD_MODNAME, chip)) {