Fix a nasty race condition in CCA determination on CC2420

We now ensure that CS signal is active, and interrupts are disabled, when querying CC2420 status byte for CCA.
Also do a little refactoring in the same cc2420_get_cca function (renamed c variable into count, and named the constant corresponding to RSSI "timeout"), so that all the driver_cc2420 C files have the same function.