UBSan: Use typecast to avoid unsigned integer overflow

UBSan: Use typecast to avoid unsigned integer overflow

iface->num_bss is unsigned integer, so need to explicit typecast it to
unsigned before decrementation by one even when the result is stored in
an unsigned integer.

../src/ap/hostapd.c:2185:26: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned long'

Signed-off-by: Jouni Malinen <j@w1.fi>