Bluetooth: BR_SMP: Add a flag BT_CONN_BR_PAIRED

Due to the BT_CONN_BR_PAIRING is cleared when
receiving ssp pairing complete event. The LTK
key cannot be derived after the BR connection
encrypted.

Add a flag BT_CONN_BR_PAIRED that the pairing
has been done.

Use this flag to indicate whether the LTK
derivation needs to be applied if the BR ACL
connection is encrypted.

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