net: usb: lan78xx: Add error handling to lan78xx_setup_irq_domain

Update `lan78xx_setup_irq_domain` to handle errors in
`lan78xx_read_reg`.  Return the error code immediately if the read
operation fails, ensuring proper error propagation during IRQ domain
setup.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241209130751.703182-2-o.rempel@pengutronix.de


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