Fix CPU spike when pressing DTMF after agent login.

The problem here is that DTMF was being continuously deferred and requeued
since ast_safe_sleep is called in a loop. There are serveral other places in the
code that sleeps and then loops in a similar fashion. Because of this fact I
opted to not defer DTMF any more, which will not affect the original fix:

https://reviewboard.asterisk.org/r/674

(closes issue #18130)
Reported by: rgj


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@301502 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/channel.c

@@ -1246,13 +1246,13 @@ int ast_is_deferrable_frame(const struct ast_frame *frame)
 	 * be queued up or not.
 	 */
 	switch (frame->frametype) {
-	case AST_FRAME_DTMF_END:
 	case AST_FRAME_CONTROL:
 	case AST_FRAME_TEXT:
 	case AST_FRAME_IMAGE:
 	case AST_FRAME_HTML:
 		return 1;
 
+	case AST_FRAME_DTMF_END:
 	case AST_FRAME_DTMF_BEGIN:
 	case AST_FRAME_VOICE:
 	case AST_FRAME_VIDEO: