plugins/dac_data_manager.c: Fix stack corruption in get_tx_channel_state()
Corruption happens due to the size difference between bool and gboolean.
sizeof(bool) = 1 and sizeof(gboolean) = 4
gtk_tree_model_get(model, &iter, TX_CHANNEL_ACTIVE, &state, -1) expects that the TX_CHANNE_ACTIVE column to be of type gboolean.

Signed-off-by: Dan Nechita <dan.nechita@analog.com>