stmmac: drop custom_* fields from plat_stmmacenet_data

Both of these fields are unused and has been unused since they
were added 3 and 5 years ago. Drop them since they are clearly
not very useful.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>