drm/dp_mst: Make drm_dp_mst_dpcd_write() consistent with drm_dp_dpcd_write()

drm/dp_mst: Make drm_dp_mst_dpcd_write() consistent with drm_dp_dpcd_write()

Noticed this while having some problems with hubs sometimes not being
detected on the first plug. Every single dpcd read or write function
returns the number of bytes transferred on success or a negative error
code, except apparently for drm_dp_mst_dpcd_write() - which returns 0 on
success.

There's not really any good reason for this difference that I can tell,
and having the two functions give differing behavior means that
drm_dp_dpcd_write() will end up returning 0 on success for MST devices,
but the number of bytes transferred for everything else.

So, fix that and update the kernel doc.

Signed-off-by: Lyude Paul <lyude@redhat.com>
Fixes: 2f221a5efed4 ("drm/dp_mst: Add MST support to DP DPCD R/W functions")
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Mikita Lipski <mikita.lipski@amd.com>
Cc: Sean Paul <sean@poorly.run>
Link: https://patchwork.freedesktop.org/patch/msgid/20200306234923.547873-2-lyude@redhat.com
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>