asix-sigma: prepare FTDI open/close for "optional open"

asix-sigma: prepare FTDI open/close for "optional open"

Move all of the FTDI connection handling from api.c to protocol.c, and
prepare "forced" and "optional" open/close. This allows future driver
code to gracefully handle situations where FPGA registers need to get
accessed, while the caller may be inside or outside the "opened" period
of the session. This is motivated by automatic netlist type and sample
rate detection, to avoid the cost of repeated firmware uploads.