res_pjsip: Fix warning by deferring implicit type cast.

Mac doesn't like the comparison of -1 to an enum, so store the result of
ast_sip_str_to_dtmf to an int so we can check for the negative return
value.  ast_sip_str_to_dtmf returns an int so this is only delaying the
implicit type cast.

Change-Id: I0c262c1719ee951aae1f437d733a301cf5f8ad29