ath10k: store max tdls vdevs that fw can handle

Currently number of tdls vdevs supported by firmware
is hardcoded. For future usage it is stored in ath10k
structure based on defined value.

Signed-off-by: Marek Puzyniak <marek.puzyniak@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>