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