Bluetooth: eir: Fix using strlen with hdev->{dev_name,short_name}

Both dev_name and short_name are not guaranteed to be NULL terminated so
this instead use strnlen and then attempt to determine if the resulting
string needs to be truncated or not.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216018


Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>