net: eth: altera: Remove sgdmadesclen member from altera_tse_private

altera_tse_private->sgdmadesclen is always assigned assigned the same
value and never changes during runtime.  Remove the struct member and
use a new define for sizeof(struct sgdma_descrip) instead.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>