net: dc2114x: Add RX/TX rings into the private data

The RX/TX DMA descriptor rings are per-device-instance private data,
so move them into the private data.

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