at86rf230: request state change back to STATE_RX_ON while sending frame

After transmitting a frame the rf231 changes its state from
BUSY_TX back to PLL_ON. To receive frames we need to change
the state to RX_ON. Requesting the state change to RX_ON
while the radio is still in state BUSY_TX will reduce the
time needed to switch the radio back to receive mode after
the current frame was transmitted. This will eliminated
some situations where frames get lost while the radio
is switching back to RX_ON state.

Signed-off-by: Sascha Herrmann <sascha@ps.nvbi.de>