ixl: Add missing conversions from/to LE16

This fixes some errors on PPC64, during attach and when trying to assign an IP
to an interface. With this change, basic operation of X710 NICs is now
possible.

This also fixes builds with IXL_DEBUG enabled

Reviewed by: erj
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D23975