[wb_acq_core] Check lmt_curr_chan_id signal and its use

[lerwys]

Problem: Some internal modules sync lmt_curr_chan_id signal with fs_clk_i, which is wrong, as the lmt_curr_chan_id signal is sync'ed with ext_clk_i. This does not apparently causes problems, as this signal are static and changed only at the beginning of each acquisition.