Fix issue with transmitting long packets on si1060
Fix issue with transmitting long packets on si1060

It appears the TX_FIFO_ALMOST_EMPTY flag in PH_STATUS register is an
unreliable indicator of being able to insert more bytes into the fifo.
Relying on the space returned by the FIFO_INFO command instead fixes
fifo underflow/overflow errors previously observed while transmitting
long packets.

Dronecode/SiK,Dronecode/SiK,Dronecode/SiK,Dronecode/SiK