ath10k: don't use interrupts for BMI

It's not really necessary for interrupts to be
used for BMI. BMI already assumes there's only one
caller at a time and it works directly with CE.

Make BMI poll for CE completions instead of
waiting for interrupts. This makes disabling
interrupts during early boot possible.

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