Store virtual address of buffer in mvneta_rx_ring

Now the virtual address of received buffer is taken from a software ring.
Thanks to this, we can use the NETA driver on 64 bits architecture and
avoid 32-bit buf_cookie descriptor field limitation.

Submitted by: Patryk Duda <pdk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12257