drm/amd/display: Fix the checking condition in dmub aux handling

[Why & How]
Fix the checking condition for detecting AUX_RET_ERROR_PROTOCOL_ERROR.
It was wrongly checking by "not equals to"

Reviewed-by: Ray Wu <ray.wu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 1db6c9e9)
Cc: stable@vger.kernel.org