drivers: ieee802154: cc2520: Fix compiler warning

The stack address was incorrectly specified when stack usage
was being debugged. This caused compiler to emit this warning

drivers/ieee802154/ieee802154_cc2520.c:1136:16: warning:
    dereferencing type-punned pointer will break strict-aliasing
    rules [-Wstrict-aliasing]
       ((struct cc2520_context *)(&__device_cc2520))->
                ^

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>