ath10k: fixup wait_for_completion_timeout return handling

wait_for_completion_timeout does not return negative values so the tests
for <= 0 are not needed and the case differentiation in the error handling
path unnecessary.

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>