ALSA: serial-u16550: More constification

Apply const prefix to the string array.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-59-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/serial-u16550.c

@@ -42,7 +42,7 @@ MODULE_SUPPORTED_DEVICE("{{ALSA, MIDI serial u16550}}");
 #define SNDRV_SERIAL_MS124W_MB 3   /* Midiator MS-124W in M/B mode */
 #define SNDRV_SERIAL_GENERIC 4     /* Generic Interface */
 #define SNDRV_SERIAL_MAX_ADAPTOR SNDRV_SERIAL_GENERIC
-static char *adaptor_names[] = {
+static const char * const adaptor_names[] = {
 	"Soundcanvas",
         "MS-124T",
 	"MS-124W S/A",