drivers: ieee802154: fix configuring CSL IE in ACK

The code was assigning a pointer to a scoped buffer to
an object outside that scope. This would cause the driver
would receive garbage instead of a well-formatted IE and
would likely reject the IE. This in turn caused CSL IE
not being included in enhanced ACKs (verified with
wireshark).

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>