ath10k: fix missing checks for bmi reads and writes

ath10k_bmi_write32 and ath10k_bmi_read32 can fail. The fix
checks their statuses to avoid potential undefined behaviors.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>