dahdi: Constify the dahdi_txlevelnames and return of dahdi_lboname().

Also moves the dahdi_txlevelnames array into the dahdi_lboname function,
which is the only place it it used.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
Acked-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>

git-svn-id: http://svn.asterisk.org/svn/dahdi/linux/trunk@9468 a0bf4364-ded3-4de4-8d8a-66a801d63aff

include/dahdi/kernel.h

@@ -1060,7 +1060,7 @@ int dahdi_transcoder_alert(struct dahdi_transcoder_channel *ztc);
 int dahdi_unregister(struct dahdi_span *span);
 
 /*! \brief Gives a name to an LBO */
-char *dahdi_lboname(int lbo);
+const char *dahdi_lboname(int lbo);
 
 /*! \brief Tell DAHDI about changes in received rbs bits */
 void dahdi_rbsbits(struct dahdi_chan *chan, int bits);