mgcp: Initialize the tone to CHAR_MAX as this might not be a request

The RQNT message might not contain a 'S:' line with the actual tone
to play. Instead of calling the callback with the 0 as tone just leave
early.

Example:
X: 6B9519B88F0
R: D/[0-9#*](N), G/ft, fxr/t38
1 file changed
tree: 82d5f1d53c8af5f9b965b3c413fb9ae6d00c5835
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README