Add flushing check to WriteUpgradeTryChannel

[colin-axner]

Summary

In WriteUpgradeTryChannel account for the channel end already being flushed:

channel.FlushStatus = types.FLUSHING
if  !k.hasInflightPackets(ctx, portID, channelID) {
    channel.FlushStatus = types.FLUSHCOMPLETE
}

It would be nice if we could add some tests, maybe some unit tests for the WriteUpgradeTryChannel func

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