wcn36xx: Add set_rekey_data callback

Add a callback for Group Temporal Key tracking as provided by the standard
WiFi ops structure.

We track the key to integrate GTK offloading into the WoWLAN suspend path
later on. Code comes from the Intel iwlwifi driver with minimal name
changes.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Tested-by: Benjamin Li <benl@squareup.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210605011140.2004643-8-bryan.odonoghue@linaro.org