Bluetooth: controller: Add support for Read/Write Conn. Accept Timeout

Implement HCI commands HCI_Read_Connection_Accept_Timeout and
HCI_Write_Connection_Accept_Timeout, and enable the feature in
supported commands.
Remove hardcoded use of default accept timeout in new LLCP code, and
use configurable value instead.

This makes EBQ test /HCI/CIN/BV-03-C pass.

Signed-off-by: Morten Priess <mtpr@oticon.com>