drivers/at86rf231: refactoring of the at86rf231 radio driver

* deploy extended operation mode
* cleanup
* implement netdev 802154.h interface
netdev/802154: suppress cppcheck warnings in 802154.h

Suppress cppcheck warnings of class unusedStructMember because
this interface is not used yet. Remove these comments once it
is used.
Merge pull request #1846 from thomaseichinger/at86rf23x

drivers/at86rf231: refactoring of the at86rf231 radio driver