app/testpmd: cleanup tunnel protocols parsing

app/testpmd: cleanup tunnel protocols parsing

This is a cleanup commit.
It assembles all tunnel outer updates into one function call to avoid
code duplications.
It defines RTE_VXLAN_GPE_DEFAULT_PORT (4790) in accordance with all
other tunnel protocol definitions.
It replaces all numeric values 4789 in their corresponding definition
RTE_VXLAN_GPE_DEFAULT_PORT.
It updates the 'csum parse-tunnel' documentation.

Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>