res_rtp_asterisk: Add packet subtype during RTCP debug when relevant

For some RTCP packet types the report count is actually the packet's subtype.
This was not being reflected in the packet debug output.

This patch makes it so for some RTCP packet types a "Packet Subtype" is
now output in the debug replacing the "Reception reports" (i.e count).

Change-Id: Id4f4b77bb37077a4c4f039abd6a069287bfefcb8