sfxge(4): only raise an exception after MC assert or reboot in the common code

Fix efx_mcdi_request_poll so it only raises an exception if EIO is
reported from a detected MC assert or reboot. This prevents
an unnecessary exception being raised if an MCDI response error code
is trandlated to EIO.

Submitted by: Andy Moreton <amoreton at solarflare.com>
Sponsored by: Solarflare Communications, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D6392