ath10k: cleanup ath10k_pci_wait_for_target_init()

ath10k_pci_wait_for_target_init() did really follow the style used elsewhere in
ath10k. Use ath10k_pci_read/write() wrappers, simplify the while loop and
improve warning messages.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>