Bluetooth: Controller: Fix conn param req ack/req reset

llcp.conn_param.ack was reset on tx of conn param rsp, however this
should only be done once the expected conn update ind is received
and sets the cu.ack flag to indicate cu procedure 'takes over'.

Signed-off-by: Erik Brockhoff <erbr@oticon.com>
Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>