asix-sigma: Only open the USB device once (fails with newer libftdi)

The asix-sigma driver was reported to fail in combination with newer
libftdi versions, because the firmware upload routine opened again an
already opened device, and then failed to claim the interface. Which was
not fatal before with previous libftdi versions.

Remove the redundant open call. Remove the local FTDI context variable,
which brings the firmware upload routine in line with all other calls
that communicate to the USB device.

This fixes bug #471.

Suggested-By: Marian Cingel <redacted>