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