sfxge(4): fix error code usage

MCDI results returned in req.emr_rc have already been translated
from MC_CMD_ERR_* to errno names, so using an MC_CMD_ERR_* value
is incorrect.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18069