net: stmmac: fix wrongly set buffer2 valid when sph unsupport

In current driver, buffer2 available only when hardware supports split
header. Wrongly set buffer2 valid in stmmac_rx_refill when refill buffer
address. You can see that desc3 is 0x81000000 after initialization, but
turn out to be 0x83000000 after refill.

Fixes: 67afd6d1 ("net: stmmac: Add Split Header support and enable it in XGMAC cores")
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>