ath10k: reject 11b tx fragmentation configuration

Even though there's a WMI enum for fragmentation
threshold no known firmware actually implements
it. Moreover it is not possible to rely frame
fragmentation to mac80211 because firmware clears
the "more fragments" bit in frame control making
it impossible for remote devices to reassemble
frames.

Hence implement a dummy callback just to say
fragmentation isn't supported. This effectively
prevents mac80211 from doing frame fragmentation
in software.

This fixes Tx becoming broken after setting
fragmentation threshold.

Fixes: 1010ba4c ("ath10k: unregister and remove frag_threshold callback")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>