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>