bluetooth: Don't use a non-string to initialize NG_HCI_BDADDR_ANY

Explicitly use an array of 6 zeroes instead of a C string containing
nul characters.  GCC 15 warns about the truncation, but this is
cleaner regardless.

In file included from /usr/obj/.../amd64.amd64/tmp/usr/include/bluetooth.h:51,
                 from usr.sbin/virtual_oss/virtual_bt_speaker/bt_speaker.c:45:
usr.sbin/virtual_oss/virtual_bt_speaker/bt_speaker.c: In function 'register_sdp':
usr.sbin/virtual_oss/virtual_bt_speaker/bt_speaker.c:96:13: error: initializer-string for array of 'unsigned char' truncates NUL terminator but destination lacks 'nonstring' attribute (7 chars into 6 available) [-Werror=unterminated-string-initialization]
   96 |             NG_HCI_BDADDR_ANY, (const uint8_t *)&record, sizeof(record),
      |             ^~~~~~~~~~~~~~~~~

Reviewed by:	dim
Differential Revision:	https://reviews.freebsd.org/D54869

sys/netgraph/bluetooth/include/ng_hci.h

@@ -447,8 +447,8 @@ typedef struct {
 } __attribute__ ((packed)) bdaddr_t;
 typedef bdaddr_t *	bdaddr_p;
 
-/* Any BD_ADDR. Note: This is actually 7 bytes (count '\0' terminator) */
-#define NG_HCI_BDADDR_ANY	(&(const bdaddr_t){"\000\000\000\000\000\000"})
+/* Any BD_ADDR. */
+#define NG_HCI_BDADDR_ANY	(&(const bdaddr_t){ { 0, 0, 0, 0, 0, 0 } })
 
 /* HCI status return parameter */
 typedef struct {