Bluetooth: Controller: Fix CIS LLL events pending check

CIS LLL events pending was checked incorrectly using a
mayfly incorrect set to be called from ULL LOW context,
but the actual call was from ULL HIGH context.

Beside this, the code was refactored to have file static
functions after global scope functions.

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