net: usb: lan78xx: Add error handling to lan78xx_init_ltm

net: usb: lan78xx: Add error handling to lan78xx_init_ltm

Convert `lan78xx_init_ltm` to return error codes and handle errors
properly.  Previously, errors during the LTM initialization process were
not propagated, potentially leading to undetected issues. This patch
ensures:

- Errors in `lan78xx_read_reg` and `lan78xx_write_reg` are checked and
  handled.
- Errors are logged with detailed messages using `%pe` for clarity.
- The function exits immediately on error, returning the error code.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241204084142.1152696-8-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>