Bluetooth: controller: Avoid race between ULL and LLL when initiating conn

Use an "initiated" flag in the lll_conn struct to guard the processing
of PDUs related to connection initiation (CONNECT_IND,
AUX_CONNECT_RSP). This avoids races between ULL and LLL when creating
a connection.

Signed-off-by: Wolfgang Puffitsch <wopu@demant.com>