Bluetooth: Controller: Fix Tx ISO SDUs dropped due to strict SN check

Introduce a Kconfig BT_CTLR_ISOAL_SN_STRICT to relax the Tx
ISO Data SDUs being dropped due to strict check of sequence
numbers in ISOAL, i.e. dropped when ISO Data SDUs are
delayed from upper layer with respect to the current
payload number in the ISO radio events.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>