net: enetc: squash enetc_alloc_cbdr and enetc_setup_cbdr

enetc_alloc_cbdr and enetc_setup_cbdr are always called one after
another, so we can simplify the callers and make enetc_setup_cbdr do
everything that's needed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>