drm/dp_mst: fix drm_dp_dpcd_read return value checks

drm_dp_dpcd_read returns the number of bytes read. The previous code
would print garbage on DPCD error, and would exit with on error on
success.

Signed-off-by: Simon Ser <contact@emersion.fr>
Fixes: cb897542 ("drm/dp_mst: Fix W=1 warnings")
Cc: Lyude Paul <lyude@redhat.com>
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/473500/