net: dc2114x: Pass private data around

This patch replaces the various uses of struct eth_device for accessing
device private data with struct dc2114x_priv, which is compatible both
with DM and non-DM operation.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Ramon Fried <rfried.dev@gmail.com>