net: stmmac: xgmac: Remove uneeded computation for RFA/RFD

RFA and RFD should not be dependent on FIFO size. In fact, the more FIFO
space we have, the later we can activate Flow Control. Let's use
hard-coded values for RFA and RFD for all FIFO sizes with the exception
of 4k, which is a special case.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>