net: usb: lan78xx: Fix error handling in MII read/write functions

Ensure proper error handling in `lan78xx_mdiobus_read` and
`lan78xx_mdiobus_write` by checking return values of register read/write
operations and returning errors to the caller.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241204084142.1152696-6-o.rempel@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>