net/mvpp2: fix build with gcc 10

GCC 10 is detecting the enum mismatch when assigning UDP variables
with MUSDK constants for TCP.

drivers/net/mvpp2/mrvl_flow.c:2521:47: error: implicit conversion
from 'enum mv_net_tcp_fields' to 'enum mv_net_udp_fields'
[-Werror=enum-conversion]

An assigned field is also fixed from "tcp" to "udp".

Fixes: 7235341d7517 ("net/mrvl: support classifier")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Liron Himi <lironh@marvell.com>
Acked-by: David Marchand <david.marchand@redhat.com>
Acked-by: Kevin Traynor <ktraynor@redhat.com>