ath10k: call correct function for frag threshold

Rts threshold was being configured instead of
fragmentation threshold.

Keep in mind available firmware binaries don't
seem to support fragmentation anyway so this
doesn't fix fragmentation threshold per se.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>