Altera TSE: Fix DMA secriptor length initialization

sgdma_descrip is a function name as well as the name of a struct. In
sgdma_initialize(), we should initialize the descriptor length field
with the actual length of a descriptor not with the size of the
function. In order to prevent such things from happening in the future,
rename the function to sgdma_setup_descrip().

Found by sparse which yields the following warning:

drivers/net/ethernet/altera/altera_sgdma.c:74:30: warning: expression using sizeof on a function

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