wl12xx: init CMD_TEMPL_KLV to sizeof(ieee80211_qos_hdr)

The keep alive template should have a max size of
sizeof(struct ieee80211_qos_hdr).

Additionally, Remove the redundant wl12xx_qos_null_data_template
struct.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>