net: usb: lan78xx: Add error handling to set_rx_max_frame_length and set_mtu

Improve error handling in `lan78xx_set_rx_max_frame_length` by:
- Checking return values from register read/write operations and
  propagating errors.
- Exiting immediately on failure to ensure proper error reporting.

In `lan78xx_change_mtu`, log errors when changing MTU fails, using `%pe`
for clear error representation.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241204084142.1152696-9-o.rempel@pengutronix.de


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