ath9k: Reset keycache on resume

It looks like some hardware registers are left into undefined state
after suspend/resume. At minimum, this can cause odd issues related to
key cache and hardware trying to encrypt/decrypt frames unexpectedly.
This seems to happen even when there is no keys configured, i.e., hardware
can end up touching TX frames just based of invalid key cache context
even if the driver is not asking a specific entry to be used. In
addition, RX can likely be affected. This patch fixes this issue.

Signed-off-by: Jouni Malinen <Jouni.Malinen@Atheros.com>
Signed-off-by: Mohammed Shafi Shajakhan <mshajakhan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>