staging: wilc1000: change return type to 'void' tcp ack filter functions

Use 'void' return type for below functions as they always return '0' and
their return value is not used by caller.
add_tcp_pending_ack()
update_tcp_session()
add_tcp_pending_ack()

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>