ath9k: use iowrite32 over __raw_writel

This patch changes the ath9k_pci_owl_loader to use the
same iowrite32 memory accessor that ath9k_pci is using
to communicate with the PCI(e) chip.

This will fix endian issues that came up during testing
with loaned AVM Fritz!Box 7360 (Lantiq MIPS SoCs + AR9287).

Fixes: 5a4f2040 ("ath9k: add loader for AR92XX (and older) pci(e)")
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>