ath9k: remove a bogus WARN_ON

On embedded hardware it's normal to not have a PCI device for the PCI
bridge that the wifi card is attached to. pdev->bus->self will be
NULL in that case. In that case, simply return without emitting an
useless kernel stack trace.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>