Bluetooth: ecdh_helper - reveal error codes

ecdh_helper functions were hiding the error codes and chose to return
the return value of an relational operator, "==". Remove the unnecessary
query and reveal the error codes.

While updating the return values, code in a way that compilers will
warn in case of uninitialized err.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>