wcn36xx: don't pad beacons for mesh

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: Jason Mobarak <jam@cozybit.com>
Signed-off-by: Chun-Yeow Yeoh <yeohchunyeow@cozybit.com>