Check Firmware Version before enabling Levels menu item

The level meters don't work correctly on earlier kernel versions, so
they shouldn't be displayed in that case. The "Firmware Version" ALSA
control was added to the kernel at the same time as the level meters
were fixed, so use the presence of that to determine whether or not to
enable the menu item.

src/alsa.c

@@ -470,6 +470,14 @@ static void alsa_add_card_callback(struct alsa_card *card) {
   );
 }
 
+static void alsa_get_firmware_version(struct alsa_card *card) {
+  struct alsa_elem *elem = get_elem_by_name(card->elems, "Firmware Version");
+
+  if (!elem)
+    return;
+  card->firmware_version = alsa_get_elem_value(elem);
+}
+
 static void alsa_subscribe(struct alsa_card *card) {
   int count = snd_ctl_poll_descriptors_count(card->handle);
 
@@ -524,6 +532,7 @@ void alsa_scan_cards(void) {
     card->handle = ctl;
 
     alsa_get_elem_list(card);
+    alsa_get_firmware_version(card);
     alsa_subscribe(card);
 
     create_card_window(card);