net: fec_mxc: Fix DM driver issue in recv

When using ethernet DM driver, the recv interface has a
change with non-DM interface, that driver needs to set
the packet pointer and provide it to upper layer to process.

In fec driver, the fecmxc_recv functions does not handle the
packet pointer parameter. This may cause crash in upper layer
processing because the packet pointer is not set.

This patch allocates a buffer for the packet pointer and free it
through free_pkt interface.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>