net: cdc_mbim: send ZLP after max sized NTBs

We normally avoid sending ZLPs by padding NTBs with a zero byte
if the NTB is shorter than dwNtbOutMaxSize, resulting in a short
USB packet instead of a ZLP.  But in the case where the NTB length
is exactly dwNtbOutMaxSize and this is an exact multiplum of
wMaxPacketSize, then we must send a ZLP.

This fixes an issue seen on a Sierra Wireless MC7710 device
where the transmission would fail whenever we ended up padding
the NTBs to max size.

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

drivers/net/usb/cdc_mbim.c

@@ -366,7 +366,7 @@ err:
 
 static const struct driver_info cdc_mbim_info = {
 	.description = "CDC MBIM",
-	.flags = FLAG_NO_SETINT | FLAG_MULTI_PACKET | FLAG_WWAN,
+	.flags = FLAG_NO_SETINT | FLAG_MULTI_PACKET | FLAG_WWAN | FLAG_SEND_ZLP,
 	.bind = cdc_mbim_bind,
 	.unbind = cdc_mbim_unbind,
 	.manage_power = cdc_mbim_manage_power,