net: wan: farsync: use new tasklet API

net: wan: farsync: use new tasklet API

This converts the driver to use the new tasklet API introduced in
commit ad117fab14f5 ("tasklet: Introduce new initialization API")

The new API changes the argument passed to callback functions,
but fortunately it is unused so it is straight forward to use
DECLARE_TASKLET rather than DECLARE_TASLKLET_OLD.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20210204173947.92884-1-kernel@esmil.dk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>