net: Fix warning when including netdev.h on DM systems

If the DM_ETH is enabled and netdev.h is included somewhere, the
struct eth_device may not be defined, yet it is used in the header
file as an argument to fecmxc_register_mii_postcall. Add forward
declaration to remove the warning.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>