crypto: picoxcell - adjust the position of tasklet_init and fix missed tasklet_kill

Since tasklet is needed to be initialized before registering IRQ
handler, adjust the position of tasklet_init to fix the wrong order.

Besides, to fix the missed tasklet_kill, this patch adds a helper
function and uses devm_add_action to kill the tasklet automatically.

Fixes: ce921368 ("crypto: picoxcell - add support for the picoxcell crypto engines")
Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>