tty: n_gsm: fix encoding of control signal octet bit DV

n_gsm is based on the 3GPP 07.010 and its newer version is the 3GPP 27.010.
See https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1516
The changes from 07.010 to 27.010 are non-functional. Therefore, I refer to
the newer 27.010 here. Chapter 5.4.6.3.7 describes the encoding of the
control signal octet used by the MSC (modem status command). The same
encoding is also used in convergence layer type 2 as described in chapter
5.5.2. Table 7 and 24 both require the DV (data valid) bit to be set 1 for
outgoing control signal octets sent by the DTE (data terminal equipment),
i.e. for the initiator side.
Currently, the DV bit is only set if CD (carrier detect) is on, regardless
of the side.

This patch fixes this behavior by setting the DV bit on the initiator side
unconditionally.

Fixes: e1eaea46bb ("tty: n_gsm line discipline")
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220218073123.2121-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -439,7 +439,7 @@ static u8 gsm_encode_modem(const struct gsm_dlci *dlci)
 		modembits |= MDM_RTR;
 	if (dlci->modem_tx & TIOCM_RI)
 		modembits |= MDM_IC;
-	if (dlci->modem_tx & TIOCM_CD)
+	if (dlci->modem_tx & TIOCM_CD || dlci->gsm->initiator)
 		modembits |= MDM_DV;
 	return modembits;
 }