Improved Radio Buffering and Initialization - #19

The radio TX FIFO was appending only 4 bytes at a time because the FIFO would sometimes overflow when adding bigger chunks. This change resolves that issue by adding hysteresis to the almost empty bit polling, which gets updated only with the TX clock of the radio.

Disabling the RX interrupt and switching to PLL tune mode was apparently causing intermittent data loss. A ping pong buffer makes it possible to leave the interrupt enabled such that the TDM loop can operate on one buffer while the RX interrupt operates on the other. Since the program does not copy any data, the buffer switching is efficient in terms of performance.

Sometimes after issuing ATZ the radio would fail to initialize properly; a fix is included to check the radio version only after the chip asserts CHIPRDY. The EZRADIOPRO Detailed Register Descriptions (AN440 rev 0.7, page 12) indicate that SPI operations should occur only after this occurs.