ath9k_htc: Fix warning on device removal

The commit "ath9k_hw: warn if we cannot change the power to the chip"
introduced a new warning to indicate chip powerup failures, but this
is not required for devices that have been removed. Handle USB device
removal properly by checking for unplugged status.

For PCI devices, this warning will still be seen when the card is pulled
out, not sure how to check for card removal.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>