net: stmmac: Use correct values in TQS/RQS fields

Currently we are using all the available fifo size in RQS and
TQS fields. This will not work correctly in multi-queues IP's
because total fifo size must be splitted to the enabled queues.

Correct this by computing the available fifo size per queue and
setting the right value in TQS and RQS fields.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>