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.

Squashed with fix from Jason Mobarak <jam@cozybit.com>:
Patch "wcn36xx: Pad TIM PVM if needed" has caused a regression in mesh
beaconing.  The field tim_off is always 0 for mesh mode, and thus
pvm_len (referring to the TIM length field) and pad are both incorrectly
calculated.  Thus, msg_body.beacon_length is incorrectly calculated for
mesh mode. Fix this.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
Signed-off-by: Jason Mobarak <jam@cozybit.com>
[bjorn: squashed in Jason's fixup]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>