Saving data to file in z1-sampler.c breaks networking

[heliosfa]

Saving data to a file as part of sampling causes the networking to break (temporarily or permanently).

The node either:
stops responding to the Port 1 interrupt, the CC1120 triggers with MCU_WAKEUP, so RX in never dealt with and the RXFIFO on the CC1120 overflows, or;

mis-reads the rssi0 register from the CC1120 as "clear" when there is traffic so turns off the radio when it should be kept on. Snooping traffic with a logic analyser shows that the CC1120 is sending the correct value to say that the channel is not clear.