drm/msm: fixup wait_for_completion_timeout handling

wait_for_completion_timeout return >= 0 but never negative so the check
logic looks inconsistent. Further the return value of
wait_for_completion_timeout was being passed up the call chain but the
x call sites as drm_dp_i2c_do_msg()/drm_dp_dpcd_access() check for < 0
thus timeout was being treated as success case.

<snip> drivers/gpu/drm/drm_dp_helper.c:drm_dp_i2c_do_msg()
                mutex_lock(&aux->hw_mutex);
                ret = aux->transfer(aux, msg);
                mutex_unlock(&aux->hw_mutex);
                if (ret < 0) {
<snip>
logic in edp_aux_transfer() seems incorrect as it could return 0 (timeout)
but checks of <= 0 to indicate error so the return probably should be
-ETIMEDOUT in case wait_for_completion_timeout returns 0 (timeout
occurred).
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>

drivers/gpu/drm/msm/edp/edp_aux.c

@@ -119,6 +119,7 @@ ssize_t edp_aux_transfer(struct drm_dp_aux *drm_aux, struct drm_dp_aux_msg *msg)
 {
 	struct edp_aux *aux = to_edp_aux(drm_aux);
 	ssize_t ret;
+	unsigned long time_left;
 	bool native = msg->request & (DP_AUX_NATIVE_WRITE & DP_AUX_NATIVE_READ);
 	bool read = msg->request & (DP_AUX_I2C_READ & DP_AUX_NATIVE_READ);
 
@@ -147,15 +148,16 @@ ssize_t edp_aux_transfer(struct drm_dp_aux *drm_aux, struct drm_dp_aux_msg *msg)
 		goto unlock_exit;
 
 	DBG("wait_for_completion");
-	ret = wait_for_completion_timeout(&aux->msg_comp, 300);
-	if (ret <= 0) {
+	time_left = wait_for_completion_timeout(&aux->msg_comp, 300);
+	if (!time_left) {
 		/*
 		 * Clear GO and reset AUX channel
 		 * to cancel the current transaction.
 		 */
 		edp_write(aux->base + REG_EDP_AUX_TRANS_CTRL, 0);
 		msm_edp_aux_ctrl(aux, 1);
-		pr_err("%s: aux timeout, %zd\n", __func__, ret);
+		pr_err("%s: aux timeout, %lu\n", __func__, time_left);
+		ret = -ETIMEDOUT;
 		goto unlock_exit;
 	}
 	DBG("completion");