qmi_wwan: set FLAG_SEND_ZLP to avoid network initiated disconnect

It has been reported that the dummy byte we add to avoid
ZLPs can be forwarded by the modem to the PGW/GGSN, and that
some operators will drop the connection if this happens.

In theory, QMI devices are based on CDC ECM and should as such
both support ZLPs and silently ignore the dummy byte.  The latter
assumption failed.  Let's test out the first.

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