[pcnet32] Fix uninitialised variable

Reported-by: Geoff Lywood <glywood@vmware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/pcnet32.c

 pcnet32_setup_probe_phy ( struct pcnet32_private *priv )
 {
 	unsigned long ioaddr = priv->pci_dev->ioaddr;
-	char phycount;
+	unsigned int phycount = 0;
 	int phy_id;
 	int i;