stmmac: reorganize chain/ring modes removing Koptions

Previously we had two Koptions to decide if the stmmac
had to use either a ring or a chain to manage its descriptors.
This patch removes the Kernel configuration options and it allow us
to use the chain mode by passing a module option.
Ring mode continues to be the default.

Also with this patch, it will be easier to validate the driver built and
guarantee that all the two modes always compile fine.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>