net: ks8851: Pass around driver private data

Introduce a private data structure for this driver with embedded
struct eth_device and pass it around. This prepares the driver to
work with both DM and non-DM systems.

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