ADCMD_FREE is the opposite of ADCMD_ALLOCATE. When you perform ADCMD_FREE
on a channel, it does a CMD_RESET command on the hardware and "unlocks"
the channel. It also checks to see if there are other pending allocation
requests. You do not need to perform ADCMD_FREE on channels stolen from
you. If you want channels back after they have been stolen, you must
reallocate them with the same allocation key.