sfxge: cleanup: adjust efx_mcdi_get_port_modes() comment for clarity

Fix an explanatory comment which did not explain very well.

Submitted by: Richard Houldsworth <rhouldsworth at solarflare.com>
Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D4916