[le audio] Prevent creating broadcast with all 0xFF broadcast code

As suggested by BASS ES-23366, all 0xFF broadcast code should be
avoided.
Application will not set all 0xFF this because of UTF-8 format.
From stack side, we're adding this check to prevent such usage for security.

Bug: 353573907
Test: atest bluetooth_test_broadcaster
Flag: Exempt; trivial change covered with unit test
Change-Id: I6b9c549ca2a1694af6f72d1bdfe18f5b047bd479