net: usb: lan78xx: Fix memory leak on device unplug by freeing PHY device

net: usb: lan78xx: Fix memory leak on device unplug by freeing PHY device

Add calls to `phy_device_free` after `fixed_phy_unregister` to fix a
memory leak that occurs when the device is unplugged. This ensures
proper cleanup of pseudo fixed-link PHYs.

Fixes: 89b36fb5e532 ("lan78xx: Lan7801 Support for Fixed PHY")
Cc: Raghuram Chary J <raghuramchary.jallipalli@microchip.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20241116130558.1352230-2-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>