ftgmac100: Move the bulk of inits to a separate function

The link monitoring and error handling code will have to
redo the ring inits and HW setup so move the code out of
ftgmac100_open() into a dedicated function.

This forces a bit of re-ordering of ftgmac100_open() but
nothing dramatic.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>