crypto: picoxcell - Fix potential race condition bug

engine->stat_irq_thresh was initialized after device_create_file() in
the probe function, the initialization may race with call to
spacc_stat_irq_thresh_store() which updates engine->stat_irq_thresh,
therefore initialize it before creating the file in probe function.

Found by Linux Driver Verification project (linuxtesting.org).

Fixes: ce921368 ("crypto: picoxcell - add support for the...")
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Acked-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>