net: ks8851: Trim down struct ks_net

Most of the entries in the structure are useless, remove them. Inline
the rest of uses where applicable.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>