net: ag71xx: fix compile warnings

drivers/net/ethernet/atheros/ag71xx.c: In function 'ag71xx_probe':
drivers/net/ethernet/atheros/ag71xx.c:1776:30: warning: passing argument 2 of
 'of_get_phy_mode' makes pointer from integer without a cast [-Wint-conversion]
In file included from drivers/net/ethernet/atheros/ag71xx.c:33:
./include/linux/of_net.h:15:69: note: expected 'phy_interface_t *'
 {aka 'enum <anonymous> *'} but argument is of type 'int'

Fixes: 0c65b2b9

 ("net: of_get_phy_mode: Change API to solve int/unit warnings")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>