net: sh-eth: Remove definition of RX_DESC_SIZE and TX_DESC_SIZE

RX_DESC_SIZE and TX_DESC_SIZE mean the size of the data descriptor for sh-eth.
We can acquire this in sizeof. It is not necessary to define these in define.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>