drivers: ieee802154: improve CSL standard conformance

This change slightly simplifies the configuration of a CSL receiver and
generalized CSL_RX_TIME to EXPECTED_RX_TIME as a re-usable primitive
across several timing-sensitive IEEE 802.15.4 standard sub-protocols
(namely BE-PANs/DSME/CSL/RIT/TSCH).

This API change is based on the rules outlined in RFC #61227.

Fixes: #62918

Signed-off-by: Florian Grandel <fgrandel@code-for-humans.de>