Bluetooth: hci_bcm: Enable support for set_diag driver callback

The set_diag driver callback allows enabling and disabling the vendor
specific diagnostic information. Since Broadcom chips have support for
a dedicated LM_DIAG channel, hook it up accordingly.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>