stack/l2c: Add helper l2cu_reject_ble_coc_connection

New helper will take tL2C_CCB* as a parameter (when available).

This will be needed later on when this function will need more more knowledge.

Tag: #feature
Bug: 159786353
Test: compilation
Sponsor: jpawlowski@
Change-Id: I5b7c552c7c1421eaaead70d68ca017341b5363c7