hostap: Reduce WPA supplicant heap when MbedTLS heap is enabled

If MbedTLS uses its own heap which is a static heap (not libc heap),
then WPA supplicant heap usage will come down, so, reduce 8K (minimum
MbedTLS heap for Wi-Fi) from the libc heap size.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>
Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>