boards/z1: fix cc2420_txrx function in CC2420 driver HAL

When waiting for transmission (to CC2420) to be done,
we were wrongly waiting for UCBUSY bit to be set,
while one should actually wait for that bit to be cleared.
Merge pull request #1388 from rousselk/fix-z1-cc2420-txrx

boards/z1: fix cc2420_txrx function in CC2420 driver HAL