qmi_wwan: Add support for Quectel EG12/EM12

Quectel EG12 (module)/EM12 (M.2 card) is a Cat. 12 LTE modem. The modem
behaves in the same way as the EP06, so the "set DTR"-quirk must be
applied and the diagnostic-interface check performed. Since the
diagnostic-check now applies to more modems, I have renamed the function
from quectel_ep06_diag_detected() to quectel_diag_detected().

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>