drivers: ieee802154_cc2520: fix unified build

In the unified kernel legacy layer, both these functions are
implemented as #defines which can't be put in a struct.

Eventually this driver should be updated to use new APIs, but
this will get it to at least build which is the priority.

Change-Id: Ic5dfb5b85ff3bd0d1646f542340642549a2b2e05
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>