ath10k: don't assume this is a PCI dev in generic code

Not all devices using this driver are backed by a PCI device, so it's
not fair to assume 'to_pci_dev()' is valid. Fortunately, we only were
using this to convert straight back to a bare 'device', which means we
were only doing no-op pointer arithmetic, and not actually accessing
potentially out-of-bounds memory. But this is still bad practice.

Just use 'ar->dev' directly, since that's really all we're looking for.

Fixes: 209b2a68 ("ath10k: add platform regulatory domain support")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>