ath10k: harmonize error case handling in ath10k_core_start

All of the bringup/init functions called in ath10k_core_start return 0
on success and != 0 on failure. ath10k_wmi_wait_for_service_ready(),
ath10k_wmi_wait_for_unified_ready() and their call sites were adjusted
to fit this model.
The return type of wait_for_completion_timeout is unsigned long not int so
ath10k_wmi_wait_for_service_ready() and ath10k_wmi_wait_for_unified_ready()
were fixed up accordingly.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>