nvme: Unbreak LE builds after r329824

The parameter 'p' is unused if _BYTE_ORDER == _LITTLE_ENDIAN. Add in a
(void)p to fix the build.

sys/dev/nvme/nvme.h

@@ -1196,6 +1196,8 @@ void	nvme_le128toh(void *p)
 		tmp[i] = tmp[15-i];
 		tmp[15-i] = b;
 	}
+#else
+	(void)p;
 #endif
 }