udp: Fix receiving RTP data over multicast

Earlier, AVIO_RDWR was handled as READ, and all the checks
for the AVIO_WRONLY flag explicitly meant the write-only case.
When changed from old AVIO/URL contants to AVIO_FLAG in
59d96941f0285a5, these comparisons were updated incorrectly, by
mapping checks for AVIO_WRONLY to checks for AVIO_FLAG_WRITE.

When receiving RTP over UDP, the urlcontext is opened with
READ_WRITE flags.

This patch updates the flag comparisons to check for the same
conditions as the code did prior to 59d96941f0285a5.

Signed-off-by: Martin Storsjö <martin@martin.st>
