wcn36xx: Clean up wcn36xx_smd_send_beacon

Needed for coming improvements. No functional changes.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
[bjorn: restored BEACON_TEMPLATE_SIZE define to 0x180]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>