ath6kl: Add provision to define suspend policy in disconnected state.

It gives flexibility to the user to define suspend policy
when the suspend mode is set to WOW and the device is in
disconnected state at the time of suspend.

New module parameter wow_mode is added to get the choice
from the user. This parameter is valid only if the module
parameter suspend_mode is set to WOW.

To force WOW in connected state and cut power
in disconnected state:
  suspend_mode=0x3 wow_mode=0x1

To force WOW in connected state and deep sleep
in disconnected state (this is also the default wow_mode):
   suspend_mode=0x3 wow_mode=0x2

If there is no value specified to wow_mode during insmod,
deep sleep mode will be tried in the disconnected state.

kvalo: clarified commit log

Signed-off-by: Raja Mani <rmani@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>