ath9k: Fix descriptors for keep-alive frame

Along with AR9462, AR9565 also has an extra field
in the TX descriptor which needs to be zeroed out
for the keep alive frame. This makes the earlier
REG_WRITE redundant, so it can be removed.

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