ath10k: clean up cont frag desc init code

This makes the code easier to extend and re-use.

While at it fix _warn to _err. Other than that
there are no functional changes.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>