stmmac: dwmac-sti: refactor the init glue callbacks

Remove the two platform specific init callbacks and make
them use a common one by creating a function member in
the internal data structure. This allow us to remove the
layer of indirection and simplify the code a bit.

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