drm/dp_mst: add kernel-doc for drm_dp_mst_port.fec_capable

Fix kernel-doc warnings for drm_dp_mst_port.fec_capable.
This fixed the following warning:
drm_dp_mst_helper.h:162: warning: Function parameter or member
'fec_capable' not described in 'drm_dp_mst_port'

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: David Francis <David.Francis@amd.com>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
[Wrapped commit msg + s/network/topology]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200328132025.19910-6-sam@ravnborg.org