ath11k: Add checked value for ath11k_ahb_remove

Return value form wait_for_completion_timeout should to be checked.

This is detected by Coverity: #CID:1464479 (CHECKED_RETURN)

Fixes: d5c65159 ("ath11k: driver for Qualcomm IEEE 802.11ax devices")
Signed-off-by: Bo YU <tsu.yubo@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200621095136.7xdbzkthoxuw2qow@debian.debian-2