net: usb: lan78xx: fix possible NULL pointer dereference in lan78xx_phy_init()

net: usb: lan78xx: fix possible NULL pointer dereference in lan78xx_phy_init()

If no PHY device is found (e.g., for LAN7801 in fixed-link mode),
lan78xx_phy_init() may proceed to dereference a NULL phydev pointer,
leading to a crash.

Update the logic to perform MAC configuration first, then check for the presence
of a PHY. For the fixed-link case, set up the fixed link and return early,
bypassing any code that assumes a valid phydev pointer.

It is safe to move lan78xx_mac_prepare_for_phy() earlier because this function
only uses information from dev->interface, which is configured by
lan78xx_get_phy() beforehand. The function does not access phydev or any data
set up by later steps.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Fixes: e110bc825897 ("net: usb: lan78xx: Convert to PHYLINK for improved PHY and MAC management")
Link: https://patch.msgid.link/20250626103731.3986545-1-o.rempel@pengutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>