ath9k: Fix issues with WoW enable

This patch addresses several issues with the
ath9k_hw_wow_enable() routine:

* The usage of set/clr variables is removed. Writing
  the required values to registers is cleaner.

* The shift value of 28 for the contention window field
  in AR_WOW_PATTERN is incorrect, change it to 27.

* Disabling Keep Alive needs to be done based on the
  LINK_CHANGE option. This is done unconditionally now,
  fix this.

* The workaround for the D1/D3 issue is required only
  for AR9462.

* The bitfield for enabling pattern matching for packets
  less than 256 bytes has expanded for new chips, handle
  this accordingly.

* General cleanup.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>