app/testpmd: fix MAC endian in flow command

MAC addresses are implicitly handled in network order since they are
actually byte strings, however this is not properly enforced with MAC masks
provided as prefix lengths, which end up inverted on little endian
systems.

Fixes: 6df81b325fa4 ("app/testpmd: add items eth/vlan to flow command")
Cc: stable@dpdk.org
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>