[media] DocBook: add entry IDs for enum fe_sec_tone_mode

enum fe_sec_tone_mode is documented together with FE_SET_TONE.

Add xrefs for each entry there. This makes the hyperlinks at
frontend.h to go directly to the right documentation.
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media/dvb/fe-set-tone.xml

@@ -76,10 +76,10 @@
 	</thead>
 	<tbody valign="top">
 	<row>
-	    <entry align="char">SEC_TONE_ON</entry>
+	    <entry align="char" id="SEC-TONE-ON"><constant>SEC_TONE_ON</constant></entry>
 	    <entry align="char">Sends a 22kHz tone burst to the antenna</entry>
 	</row><row>
-	    <entry align="char">SEC_TONE_OFF</entry>
+	    <entry align="char" id="SEC-TONE-OFF"><constant>SEC_TONE_OFF</constant></entry>
 	    <entry align="char">Don't send a 22kHz tone to the antenna
 		(except if the FE_DISEQC_* ioctls are called)</entry>
 	</row>