Bluetooth: controller: fixing slot reservation for connection setup

With introduction of refactored LLCP was also introduced an error
re. calculation of initial slot reservation in connection setups.

Due to this error a bigger slot was reserved, leading to failure due to
connection overlaps in multiple.sh test.
Also the removal of this 'overhead' in slot reservations calls for
bigger time-allowance for l2pcap_stress test.

Signed-off-by: Erik Brockhoff <erbr@oticon.com>