hostap: Increase the stack size for net_mgmt

Due to recent changes in hostap to move few buffers from heap->stack (to
improve the reliability of allocations) this has increased the stack
size and with another recent change to increase the max polling sockets
this puts over the 4096 stack size for net_mgmt causing stack overflow.

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