net: usb: lan78xx: remove PHY register access from ethtool get_regs

Remove PHY register handling from `lan78xx_get_regs` and
`lan78xx_get_regs_len`. Since the controller can have different PHYs
attached, the first 32 registers are not universally relevant or the
most interesting. Simplify the implementation to focus on MAC and device
registers.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20241216120941.1690908-6-o.rempel@pengutronix.de


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