net: stmmac: Fix the packet count in stmmac_rx()

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