Changes

Fix IuUP RTP hdr seqnr field not incremented

Previous commit add osmo_io changed mgcp_udp_send() implementation from
"return sendto()", which is documented as:
"return the number of bytes sent. Otherwise, -1 shall be returned"

to "return mgcp_udp_send_msg()", which in turn calls
"return osmo_iofd_sendto_msgb()", and which is documented as:
"\returns 0 in case of success (takes msgb ownership), -1 on error
(doesn't take msgb ownership)."

So successful return code changed from >0 (bytes sent) to ==0,
but forgot to update mgcp_send_iuup() return code path check (and also
some related function documentation calling mgcp_udp_send()".

This commit fixes all the related aspects of that return code change.

Fixes: 352b967d1bee4b3bc51ecc383306bad0fc5b7fbf
Change-Id: I154e1e41cd02fd4d9b88ad98fc7c4d657246c589