altera_tse: Correct typo in obtaining tx_fifo_depth from devicetree

This patch corrects a typo in the way tx_fifo_depth is read from the
devicetree. This patch was submitted by Vlastimil about a week ago,
and is now cleaned up and resubmitted.

Signed-off-by: Vlastimil Setka <setka@vsis.cz>
Signed-off-by: Vince Bridgers <vbridger@opensource.altera.com>
Signed-off-by: David S. Miller <davem@davemloft.net>