Bluetooth: BR: SMP: Check if remote supports CID 0x0007

Add a function bt_l2cap_br_get_remote_fixed_chan to get the remote fixed
channels.

If the fixed channel CID 0x0007 is unsupported, skip the LTK derivation.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>