at86rf2xx: fix receive before send detection
at86rf2xx: fix receive before send detection

The at86rf2xx radio handled a transfer complete condition with the radio
in the BUSY_TX_ARET state as a finished transmission. This condition and
state also occurs when a reception occurs just before switching to
transmitting. This would cause a condition where first a TX_COMPLETE was
signalled and second a RX_COMPLETE was signalled. The network stack
would then read the transmitted frame as a received frame.

The patch fixes the errornous RX callback by only submitting the
TX_COMPLETE condition when there are at least 2 frames pending
(at86rf2xx::pending_tx).

basilfx/RIOT,authmillenon/RIOT,ant9000/RIOT,smlng/RIOT,miri64/RIOT,kYc0o/RIOT,yogo1212/RIOT,ant9000/RIOT,RIOT-OS/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,jasonatran/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,kYc0o/RIOT,ant9000/RIOT,miri64/RIOT,OlegHahm/RIOT,authmillenon/RIOT,miri64/RIOT,OTAkeys/RIOT,smlng/RIOT,kaspar030/RIOT,OTAkeys/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,basilfx/RIOT,yogo1212/RIOT,kaspar030/RIOT,smlng/RIOT,jasonatran/RIOT,smlng/RIOT,OTAkeys/RIOT,kYc0o/RIOT,ant9000/RIOT,basilfx/RIOT,OlegHahm/RIOT,yogo1212/RIOT,yogo1212/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,ant9000/RIOT,kYc0o/RIOT,kaspar030/RIOT,yogo1212/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,basilfx/RIOT,yogo1212/RIOT,smlng/RIOT,authmillenon/RIOT,kaspar030/RIOT,basilfx/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,OTAkeys/RIOT,OTAkeys/RIOT