sfxge(4): ignore error in completion event on MCDIv2 HW

With MCDIv2, the reponse length can be to big to fit into the
CMDDONE_DATALEN field in the MCDI completion event. But rather that
the length being truncated, it can overflow into the CMDDONE_ERRNO
field (this is a longstanding firmware bug). Hence the CMDDONE_ERRNO
field may not be valid.

It isn't necessary to use the value in the CMDDONE_ERRNO field though,
so it can be ignored. The actual error code is already read from the
response header on MCDIv2 capable hardware and stored in emr_rc, so
that can be used instead.

Submitted by: Mark Spender <mspender at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
Differential Revision: https://reviews.freebsd.org/D18104