net: stmmac: Determine earlier the size of RX buffer

Split Header feature needs to know the size of RX buffer but current
code is determining it too late. Fix this by moving the RX buffer
computation to earlier stage.

Changes from v2:
- Do not try to align already aligned buffer size

Fixes: 67afd6d1 ("net: stmmac: Add Split Header support and enable it in XGMAC cores")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>