stmmac: Configure Flow Control to work correctly based on rxfifo size

Configure flow control correctly, and based on the receive fifo size read
as a property from the devicetree since the Synopsys stmmac fifo sizes are
configurable based on a particular chip's implementation. This patch maintains
the previous incorrect behavior unless the receive fifo size is found in the
devicetree.

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