drm/msm/dp: replace variable err with len at dp_aux_link_power_up()

drm_dp_dpcd_readb() will return 1 to indicate one byte had been read
successfully. This patch replace variable "err" with "len" have more
correct meaning.

changes in v5:
-- split into 3 patches

Signed-off-by: Kuogee Hsieh <quic_khsieh@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/502533/
Link: https://lore.kernel.org/r/1662999830-13916-3-git-send-email-quic_khsieh@quicinc.com


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>