net: enetc: set up RX ring indices from enetc_setup_rxbdr()

There is only one place which needs to set up indices in the RX ring.
Be consistent with what was done in the TX path and do this in
enetc_setup_rxbdr().

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