ixl(4): Fix two important RSS bugs.

• Change tc_mapping field to assign 64 queues instead of 16 to the PF's VSI; add comments to describe how this is done.

• Set hash lut size to 512 when setting filter control; the lut size defaults to 128 if this isn't set.

Differential Revision: https://reviews.freebsd.org/D5203
Reviewed by: gallatin
Tested by: jeffrey.e.pieper@intel.com
Sponsored by: Intel Corporation