net: lan78xx: fix build errors when linux/phy*.h is removed from net/dsa.h

drivers/net/usb/lan78xx.c:394:33: sparse: expected ; at end of declaration
drivers/net/usb/lan78xx.c:394:33: sparse: Expected } at end of struct-union-enum-specifier
drivers/net/usb/lan78xx.c:394:33: sparse: got interface
drivers/net/usb/lan78xx.c:403:1: sparse: Expected ; at the end of type declaration
drivers/net/usb/lan78xx.c:403:1: sparse: got }

Add linux/phy.h to lan78xx.c

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>