docs: networking: nfc: change headlines to sphinx syntax

The headlines in this file do are not in the standard kernel docu-
mentation headline format. Change it, so this file can be switched to
rst in the future.
Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

Documentation/networking/nfc.txt

+===================
 Linux NFC subsystem
 ===================
 
@@ -8,7 +9,7 @@ This document covers the architecture overview, the device driver interface
 description and the userspace interface description.
 
 Architecture overview
----------------------
+=====================
 
 The NFC subsystem is responsible for:
       - NFC adapters management;
@@ -51,7 +52,7 @@ PF_NFC. The NFC_SOCKPROTO_RAW performs raw communication with NFC targets.
                                     +-----------+
 
 Device Driver Interface
------------------------
+=======================
 
 When registering on the NFC subsystem, the device driver must inform the core
 of the set of supported NFC protocols and the set of ops callbacks. The ops
@@ -64,7 +65,7 @@ callbacks that must be implemented are the following:
 * data_exchange - send data and receive the response (transceive operation)
 
 Userspace interface
---------------------
+===================
 
 The userspace interface is divided in control operations and low-level data
 exchange operation.