ath6kl: ignore WMI_TXE_NOTIFY_EVENTID based on fw capability flags

Certain 6004 firmware releases redefine the WMI_TXE_NOTIFY_EVENTID event
number and sends the new event frequently. However it doesn't have the
tx-err-notify feature and thus this firmware capability flag isn't set on
the firmware package. By guarding the processing of this event by the same
method we guard the sending of the WMI_SET_TXE_NOTIFY_CMDID command, we
can ignore the spurious event that we don't know how to process.

Without this change we call cfg80211_cqm_txe_notify() with possibly bad
data.

Signed-off-by: Steve deRosier <steve.derosier@lairdtech.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>