ath10k: remove send completion validation in diag read/write

CE diag window access is serialized (it has to be by design) so
there's no way to get a different send completion. so there's no
need for post completion validation.

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>