net: usb: lan78xx: Sanitize return values of register read/write functions

usb_control_msg() returns the number of transferred bytes or a negative
error code. The current implementation propagates the transferred byte
count, which is unintended. This affects code paths that assume a
boolean success/failure check, such as the EEPROM detection logic.

Fix this by ensuring lan78xx_read_reg() and lan78xx_write_reg() return
only 0 on success and preserve negative error codes.

This approach is consistent with existing usage, as the transferred byte
count is not explicitly checked elsewhere.

Fixes: 8b1b2ca ("net: usb: lan78xx: Improve error handling in EEPROM and OTP operations")
Reported-by: Mark Brown <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]
Signed-off-by: Oleksij Rempel <[email protected]>
Tested-by: Mark Brown <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: olerem

drivers/net/usb/lan78xx.c

@@ -627,7 +627,7 @@ static int lan78xx_read_reg(struct lan78xx_net *dev, u32 index, u32 *data)
 
 	kfree(buf);
 
-	return ret;
+	return ret < 0 ? ret : 0;
 }
 
 static int lan78xx_write_reg(struct lan78xx_net *dev, u32 index, u32 data)
@@ -658,7 +658,7 @@ static int lan78xx_write_reg(struct lan78xx_net *dev, u32 index, u32 data)
 
 	kfree(buf);
 
-	return ret;
+	return ret < 0 ? ret : 0;
 }
 
 static int lan78xx_update_reg(struct lan78xx_net *dev, u32 reg, u32 mask,