ftgmac100: Split ring alloc, init and rx buffer alloc

Currently, a single function is used to allocate the rings
themselves, initialize them, populate the rx ring, and
allocate the rx buffers. The same happens on free.

This splits them into separate functions. This will be
useful when properly implementing re-initialization on
link changes and error handling when the rings will be
repopulated but not freed.

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