asix-sigma: Propagate errors from firmware upload

The firmware upload code paths in the asix-sigma driver used to return
either the SR_OK code, or the magic number 0 for error conditions. Which
happens to be identical and cannot be told apart by callers.

Provide proper SR_ERR return codes for error conditions, such that
callers can tell whether the firmware upload succeeded.

This fixes part of bug #471.

Suggested-By: Marian Cingel <redacted>