net: macb: remove is_udp variable

Remove is_udp variable that is used in only one place and use
ip_hdr(skb)->protocol == IPPROTO_UDP check instead.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>