Bluetooth: nordic lll: deduplicate CIS flush-timeout paths via shared helpers

[Copilot]

Central and peripheral CIS LLL paths duplicated the same flush-timeout mechanics (subevent-limit math, Tx/Rx BN advancement, and Rx lost-data reporting) across mid-event flush, event-close finalization, and lazy-update flows. This change factors those repeated paths into shared helpers while preserving existing timeout conditions and interleaved guards.

• Shared flush helper surface

  • Added subsys/bluetooth/controller/ll_sw/nordic/lll/lll_conn_iso_flush.h.
  • Introduced:
    • lll_flush_subevent_limit(nse, bn, payload_count)
    • lll_flush_tx_advance(cis_lll)
    • lll_flush_rx_advance(cis_lll)
    • shared Rx lost-data body helper used by both roles.

• Central ISO refactor (lll_central_iso.c)

  • Replaced inline u formula at flush sites with lll_flush_subevent_limit(...).
  • Replaced repeated Tx/Rx BN advancement sequences with lll_flush_tx_advance(...) / lll_flush_rx_advance(...).
  • Reduced local iso_rx_data_lost() to role-specific timestamp derivation + shared lost-data helper call.

• Peripheral ISO refactor (lll_peripheral_iso.c)

  • Applied the same helper substitutions in:
    • payload_count_flush()
    • payload_count_rx_flush_or_txrx_inc()
    • payload_count_lazy()
  • Reduced local iso_rx_data_lost() similarly to timestamp derivation + shared helper invocation.

Example of the call-site simplification:

payload_count = cis_lll->rx.payload_count + cis_lll->rx.bn_curr - 1U;
u = lll_flush_subevent_limit(cis_lll->nse, cis_lll->rx.bn, payload_count);

if (/* existing flush condition unchanged */) {
	lll_flush_rx_advance(cis_lll);
}