drivers: i2s_nrfx: Fix a few minor fixes

- correct the names of buffers used by message queues so that it
  is possible to have multiple instances of the driver (in case
  such need appears in the future)
- make `stop` and `discard_rx` normal structure members, not bit
  fields, as they are modified in the interrupt handler and that
  could lead to overwriting of other bit fields located in the
  same memory unit
- add a log message providing the actual frame clock (WS) frequency
  (i.e. PCM rate) that the driver was able to configure (due to
  hardware limitations, it is not always possible to achieve the
  exact requested frequency and the driver selects the closest one
  available, so make it more visible to users

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>