api: Disable api_net when DM is used

When CONFIG_API is selected with DM_ETH this
error is present:
api/api_net.c: In function 'dev_enum_net':
api/api_net.c:61:35: warning: initialization from incompatible pointer
type
  struct eth_device *eth_current = eth_get_dev();
                                   ^
api/api_net.c:68:39: error: dereferencing pointer to incomplete type
  memcpy(di->di_net.hwaddr, eth_current->enetaddr, 6);
                                       ^
Disable api_net functions when ETH_DM is selected.

Signed-off-by: Chris Johns <chrisj@rtems.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>