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.rst

@@ -1,4 +1,10 @@
-			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.