Currently, stmmac_rx() is counting the number of descriptors but it
should count the number of packets as specified by the NAPI limit.

Fix this.

Fixes: ec222003 ("net: stmmac: Prepare to add Split Header support")
Signed-off-by: NJose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>