net:enetc: command BD ring data memory alloc as one function alone

Separate the CBDR data memory alloc standalone. It is convenient for
other part loading, for example the ENETC QOS part.

Reported-and-suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Po Liu <po.liu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>