drivers/encx24j600: Implemented missing drop case

The netdev_driver_t::recv implementation of the encx24j600 does not provide the
drop feature. This commit adds it.

Fixes: RIOT-OS#10410

drivers/encx24j600/encx24j600.c

@@ -357,7 +357,10 @@ static int _recv(netdev_t *netdev, void *buf, size_t len, void *info)
         }
         /* read packet (without 4 bytes checksum) */
         sram_op(dev, ENC_RRXDATA, 0xFFFF, buf, payload_len);
+    }
 
+    /* Frame was retrieved or drop was requested --> remove it from buffer */
+    if (buf || (len > 0)) {
         /* decrement available packet count */
         cmd(dev, ENC_SETPKTDEC);