osmo_ortp: Don't print ERROR message for every missing RTP frame
osmo_ortp: Don't print ERROR message for every missing RTP frame

* it's inconsistent to print ERROR in a LOGL_INFO message
* loosing RTP packets is not that unusual, especially when the channel
  is just starting up

Change-Id: Ide0017987b4cea7fd24795941494f1da913ea673
Closes: OS#4464

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis