net: cdc_ncm: set reasonable padding limits

We pad frames larger than X to maximum size for devices which
don't need a ZLP after maximum sized frames. This allows the
device to optimize its transfers for one fixed buffer size.

X was arbitrarily set at 512 bytes regardless of real buffer
maximum, causing extreme overheads due to excessive padding of
larger tx buffers. Limit the padding to at most 3 full USB
packets, still allowing the overhead to payload ratio of 3/1.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/usb/cdc_ncm.h

@@ -115,6 +115,7 @@ struct cdc_ncm_ctx {
 	u16 tx_seq;
 	u16 rx_seq;
 	u16 connected;
+	u16 min_tx_pkt;
 };
 
 u8 cdc_ncm_select_altsetting(struct usb_interface *intf);