[nrf fromtree] Bluetooth: Conn: add connection type helper functions

Introduce dedicated helper functions for connection type checking:
- bt_conn_is_br() for BR/EDR connections
- bt_conn_is_le() for LE connections
- bt_conn_is_iso() for ISO connections
- bt_conn_is_sco() for SCO connections

Replace direct conn->type comparisons with these new helper functions
throughout the connection management code. This improves code readability,
maintainability, and provides proper configuration checks for each
connection type.

Signed-off-by: Kai Cheng <[email protected]>
(cherry picked from commit 038523c)

Author: chengkai15