sfxge: cleanup: quieten efx_mcdi_read_resonse_header error reporting

The "mcdi_err_arg" probe still reports results of failed MCDI
commands, unless the caller invoked efx_mcdi_execute_quiet().

Submitted by: Andy Moreton <amoreton at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4919