Get FREEDV_MODE_FSK_LDPC working with freedv data API and small fix to README_data.md

[rafael2k]

No description provided.

[drowe67]

@rafael2k - can you pls provide some explanation for this PR:

1. What was broken?
2. What have you fixed?
3. How can this be demonstrated (e.g. a test)?

[rafael2k]

Hi @drowe67 !
I was getting no sync through freedv data api. With this patch I started to receive them

    freedv_get_modem_stats(freedv, &sync, &snr_est);
    if (!sync)
      ....

In the code I'm working on, here:
https://github.com/Rhizomatica/hermes-modem/blob/9fba74ac7863e97df2e7c25afe082e75ed0a2cca/modem/modem.c#L727

./mercury -r -m 6 (in one end)
./mercury -t -m 6 (on tx side, needs to key the radio)

basically I was getting wrong sync info from freedv_get_modem_stats()