bluetooth: increment `BT_BUF_CMD_TX_COUNT`

The extended advertising start procedure can consume both command
buffers in a single API call, resulting in `bt_le_create_conn_cancel`
being unable to claim a buffer to terminate the connection request.

Increase the command count if both extended advertising and Bluetooth
central are enabled in an application.

Signed-off-by: Jordan Yates <jordan@embeint.com>