ALSA: hdspm: fix spelling mistake "Externel" -> "External"

Trivial fix to spelling mistake in dev_warn message.
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/rme9652/hdspm.c

@@ -1666,7 +1666,7 @@ static int hdspm_set_rate(struct hdspm * hdspm, int rate, int called_internally)
 			    HDSPM_AUTOSYNC_FROM_NONE) {
 
 				dev_warn(hdspm->card->dev,
-					 "Detected no Externel Sync\n");
+					 "Detected no External Sync\n");
 				not_set = 1;
 
 			} else if (rate != external_freq) {