net: usb: lan78xx: Extract PHY interrupt acknowledgment to helper

Move the PHY interrupt acknowledgment logic from lan78xx_link_reset()
to a new helper function lan78xx_phy_int_ack(). This simplifies the
code and prepares for reusing the acknowledgment logic independently
from the full link reset process, such as when using phylink.

No functional change intended.

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>