asix-sigma: improve robustness of firmware download, delay and retry

Running several firmware uploads in quick repetition sometimes failed.
It's essential to stop the active netlist from preventing the FPGA's
getting reconfigured (FTDI to FPGA pins are so few, and shared). Delays
in a single iteration of the initiation sequence improves reliability.
Retries of the sequence are belt and suspenders on top of that.

Before the change, failure to configure was roughly one in ten. After
the change, several thousand reconfigurations passed without failure.