net: enetc: implement ring reconfiguration procedure for PTP RX timestamping

The crude enetc_stop() -> enetc_open() mechanism suffers from 2
problems:

1. improper error checking
2. it involves phylink_stop() -> phylink_start() which loses the link

Right now, the driver is prepared to offer a better alternative: a ring
reconfiguration procedure which takes the RX BD size (normal or
extended) as argument. It allocates new resources (failing if that
fails), stops the traffic, and assigns the new resources to the rings.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>