Disable the ixgbe(4) UDP 4-tuple hashing for the time being.

A mix of fragmented and non-fragmented UDP in a single stream will end up
being hashed differently, resulting in out-of-order behaviour in the receive
path.

This was done in the linux e1000 driver in 2011.

Discussed with:	jfv
Teach ixgbe(4) about rss_gethashconfig().

If RSS is enabled, ixgbe(4) will query the RSS API for the types of hashes
which should be used.  It'll then only enable hashes that are exposed via
the RSS layer.

This way it won't try to do things like enable UDP hashing if RSS explicitly
states that it isn't supported in lookups.

Tested:

* 82599EB ixgbe(4) NIC