net: usb: lan78xx: remove explicit check for missing PHY driver

RGMII timing correctness relies on the PHY providing internal delays.
This is typically ensured via PHY driver, strap pins, or PCB layout.

Explicitly checking for a PHY driver here is unnecessary and non-standard.
This logic applies to all MACs, not just LAN78xx, and should be left to
phylib, phylink, or platform configuration.

Drop the check and rely on standard subsystem behavior.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Thangaraj Samynathan <thangaraj.s@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>