tty: n_gsm: clean up implicit CR bit encoding in address field

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 5.2.1.2 describes the encoding of the
address field within the frame header. It is made up of the DLCI address,
command/response (CR) bit and EA bit.
Use the predefined CR value instead of a plain 2 in alignment to the
remaining code and to make the encoding obvious.
Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20220420101346.3315-3-daniel.starke@siemens.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -749,7 +749,7 @@ static void __gsm_data_queue(struct gsm_dlci *dlci, struct gsm_msg *msg)
 
 	*--dp = msg->ctrl;
 	if (gsm->initiator)
-		*--dp = (msg->addr << 2) | 2 | EA;
+		*--dp = (msg->addr << 2) | CR | EA;
 	else
 		*--dp = (msg->addr << 2) | EA;
 	*fcs = gsm_fcs_add_block(INIT_FCS, dp , msg->data - dp);