audio: fix handling of sample_rate and channels count.

audio: fix handling of sample_rate and channels count.

This was utterly broken for both para_write and para_audiod as
the writer didn't have a chance to see the computed value from
the filter/wav header.

Fix it by introducing two new pointers in struct writer_node_group
which point to the corresponding variables in struct
filter/check_wav_task.

Tested with 22KHz mono files.