cc2520-v2: more clean up
cc2520-v2: add a window where pending interrupts can happen

Every iteration of processExceptions, we let pending interrupts occur
before actually starting to accessing the cc2520 chip.  This makes
this driver play better with other interrupt users.

We still also let interrupts in while spending time pulling bytes
from the RXFIFO (snarfMsg).  This hasn't changed.