ath10k: split wmi stats parsing

The parsing function was rather complex. Simplify
by splitting it up into firmware branch specific
implementations.

While at it move the parsing code into wmi.c where
it belongs.

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