wifi_nm: Fix Wi-Fi interface backward compatibility

If newly introduced interface type is unset then return the first Wi-Fi
interface as a fallback, this fixes backward compatibility.

Also, add NM APIs and use them for type checks, rather than directly
using the type enumeration.

Fixes #75332.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>