net/ixgbe: remove tpid check for flow director

net/ixgbe: remove tpid check for flow director

DPDK community has several emails discussion on this topic,
these mails link is bellow:
http://dpdk.org/ml/archives/dev/2017-March/060379.html,
http://dpdk.org/ml/archives/dev/2017-March/060295.html,

items like VLAN can already have several valid "types"
(0x88a8, 0x8100, 0x9100), and who knows what will come up
in the future.

And ixgbe_flow just ignores the types when do filter configuration.
So it may be reasonable to delete the related tpid check process.

Also add some more comment log on stack explanation.

Fixes: 11777435c72 ("net/ixgbe: parse flow director filter")
Cc: stable@dpdk.org
Signed-off-by: Wei Zhao <wei.zhao1@intel.com>