if_media.c: staticize and constify ifmedia description structures used under IFMEDIA_DEBUG.

The reason for this change is to make it clear the scope of the in-kernel usage
of IFM_TYPE_DESCRIPTIONS and IFM_SUBTYPE_ETHERNET_DESCRIPTIONS macros. Also it
is somewhat better C.

Reviewed by: hselasky
Sponsored by: Mellanox Technologies
Differential revision: https://reviews.freebsd.org/D23620