drivers/at86rf231: refactoring of the at86rf231 radio driver

* deploy extended operation mode
* cleanup
* implement netdev 802154.h interface
drivers/at86rf231: 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 remote-tracking branch 'thomas/at86rf23x' into hwtimer_irq_fixup
SQUASH ME: inISR check is superfluous