qmi_wwan: add support for Quectel EC21 and EC25

The Quectel EC21 and EC25 need the same "set DTR" request as devices
based on the MDM9230 chipset, but has no USB3 support. Our best guess
is that the "set DTR" functionality depends on chipset and/or
baseband firmware generation. But USB3 is still an optional feature.

Since we cannot enable this unconditionally for all older devices, and
there doesn't appear to be anything we can use in the USB descriptors
to identify these chips, we are forced to use a device specific quirk
flag.

Reported-and-tested-by: Sebastian Sjoholm <sebastian.sjoholm@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>