wcn36xx: Clean up wcn36xx_smd_send_beacon

Needed for coming improvements. No functional changes.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
wcn36xx: Pad TIM PVM if needed

The wcn36xx FW expects a fixed size TIM PVM in the beacon template. If
supplied with a shorter than expected PVM it will overwrite the IE
following the TIM.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>