drivers: net: usb: qmi_wwan: add QMI_QUIRK_SET_DTR for Telit PID 0x1201

Telit LE920A4 uses the same pid 0x1201 of LE920, but modem
implementation is different, since it requires DTR to be set for
answering to qmi messages.

This patch replaces QMI_FIXED_INTF with QMI_QUIRK_SET_DTR: tests on
LE920 have been performed in order to verify backward compatibility.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>