docs: networking: convert x25-iface.txt to ReST

Not much to be done here:

- add SPDX header;
- adjust title markup;
- remove a tail whitespace;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -115,6 +115,7 @@ Contents:
    udplite
    vrf
    vxlan
+   x25-iface
 
 .. only::  subproject and html
 

Documentation/networking/x25-iface.txt → Documentation/networking/x25-iface.rst

-			X.25 Device Driver Interface 1.1
+.. SPDX-License-Identifier: GPL-2.0
+
+============================-
+X.25 Device Driver Interface
+============================-
+
+Version 1.1
 
 			   Jonathan Naylor 26.12.96
 
@@ -99,7 +105,7 @@ reduced by the following measures or a combination thereof:
 (1) Drivers for kernel versions 2.4.x and above should always check the
     return value of netif_rx(). If it returns NET_RX_DROP, the
     driver's LAPB protocol must not confirm reception of the frame
-    to the peer. 
+    to the peer.
     This will reliably suppress packet loss. The LAPB protocol will
     automatically cause the peer to re-transmit the dropped packet
     later.

include/uapi/linux/if_x25.h

@@ -18,7 +18,7 @@
 
 #include <linux/types.h>
 
-/* Documentation/networking/x25-iface.txt */
+/* Documentation/networking/x25-iface.rst */
 #define X25_IFACE_DATA		0x00
 #define X25_IFACE_CONNECT	0x01
 #define X25_IFACE_DISCONNECT	0x02

net/x25/Kconfig

@@ -21,7 +21,7 @@ config X25
 	  <http://docwiki.cisco.com/wiki/X.25>.
 	  Information about X.25 for Linux is contained in the files
 	  <file:Documentation/networking/x25.txt> and
-	  <file:Documentation/networking/x25-iface.txt>.
+	  <file:Documentation/networking/x25-iface.rst>.
 
 	  One connects to an X.25 network either with a dedicated network card
 	  using the X.21 protocol (not yet supported by Linux) or one can do