net: atm: pppoatm: use tasklet_init to initialize wakeup tasklet

Previously a temporary tasklet structure was initialized on the stack
using DECLARE_TASKLET_OLD() and then copied over and modified. Nothing
else in the kernel seems to use this pattern, so let's just call
tasklet_init() like everyone else.

Signed-off-by: Emil Renner Berthing <kernel@esmil.dk>
Link: https://lore.kernel.org/r/20210127173256.13954-1-kernel@esmil.dk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>