ath10k: fix rts/fragmentation threshold setup

If RTS and fragmentation threshold values are
0xFFFFFFFF they should be considered disabled and
no min/max limits must be applied.

This fixes some issues with throughput issues,
especially with VHT.

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