boards/xx/driver_cc2420: fix race condition in CCA determination

Ensure that CS is active and IRQ disabled when querying CCA
on CC2420 (cc2420_get_cca() function).
Also do a little refactoring (renamed c variable into count,
and named the constant corresponding to RSSI "timeout").
Merge pull request #1385 from rousselk/fix-cc2420-driver-cca

Fix a nasty race condition in CCA determination on CC2420