drm/bridge: analogix_dp: properly handle zero sized AUX transactions

Address only transactions without any data are valid and should not
be flagged as short transactions. Simply return the message size when
no transaction errors occured.

CC: stable@vger.kernel.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240318203925.2837689-1-l.stach@pengutronix.de

drivers/gpu/drm/bridge/analogix/analogix_dp_reg.c

@@ -1027,7 +1027,6 @@ ssize_t analogix_dp_transfer(struct analogix_dp_device *dp,
 	u32 status_reg;
 	u8 *buffer = msg->buffer;
 	unsigned int i;
-	int num_transferred = 0;
 	int ret;
 
 	/* Buffer size of AUX CH is 16 bytes */
@@ -1079,7 +1078,6 @@ ssize_t analogix_dp_transfer(struct analogix_dp_device *dp,
 			reg = buffer[i];
 			writel(reg, dp->reg_base + ANALOGIX_DP_BUF_DATA_0 +
 			       4 * i);
-			num_transferred++;
 		}
 	}
 
@@ -1127,7 +1125,6 @@ ssize_t analogix_dp_transfer(struct analogix_dp_device *dp,
 			reg = readl(dp->reg_base + ANALOGIX_DP_BUF_DATA_0 +
 				    4 * i);
 			buffer[i] = (unsigned char)reg;
-			num_transferred++;
 		}
 	}
 
@@ -1144,7 +1141,7 @@ ssize_t analogix_dp_transfer(struct analogix_dp_device *dp,
 		 (msg->request & ~DP_AUX_I2C_MOT) == DP_AUX_NATIVE_READ)
 		msg->reply = DP_AUX_NATIVE_REPLY_ACK;
 
-	return num_transferred > 0 ? num_transferred : -EBUSY;
+	return msg->size;
 
 aux_error:
 	/* if aux err happen, reset aux */