All callers ignore the return value, so simplify by not providing one.

Note that crypto_engine_exit() is typically called in a device driver's
remove path (or the error path in probe), where errors cannot be handled
anyhow.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>