smsc95xx: fix transmission where ZLP is expected

Usbnet framework assumes USB hardware doesn't handle zero length
packets, but SMSC LAN95xx requires these to be sent for correct
operation.

This patch fixes an easily reproducible tx lockup when sending a frame
that results in exactly 512 bytes in a USB transmission (e.g. a UDP
frame with 458 data bytes, due to IP headers and our USB headers).  It
adds an extra flag to usbnet for the hardware driver to indicate that
it can handle and requires the zero length packets.

This patch should not affect other usbnet users, please also consider
for -stable.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/usb/usbnet.h

@@ -89,6 +89,7 @@ struct driver_info {
 #define FLAG_FRAMING_AX 0x0040		/* AX88772/178 packets */
 #define FLAG_WLAN	0x0080		/* use "wlan%d" names */
 #define FLAG_AVOID_UNLINK_URBS 0x0100	/* don't unlink urbs at usbnet_stop() */
+#define FLAG_SEND_ZLP	0x0200		/* hw requires ZLPs are sent */
 
 
 	/* init device ... can sleep, or cause probe() failure */