stmmac: fix and better tune the default buffer sizes

This patch is to fix and tune the default buffer sizes.
It reduces the default bufsize used by the driver from
4KiB to 1536 bytes.

Patch has been tested on both ARM and SH4 platform based.

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