crypto: rockchip - fix to check return value

crypto_engine_alloc_init may fail, e.g., as result of a fail of
devm_kzalloc or kthread_create_worker. Other drivers (e.g.,
amlogic-gxl-core.c, aspeed-acry.c, aspeed-hace.c, jr.c, etc.) check
crypto_engine_alloc_init's return value and return -ENOMEM in case
a NULL pointer is returned. This patch inserts a corresponding
return value check to rk3288_crypto.c.

Signed-off-by: Kilian Zinnecker <kilian.zinnecker@mail.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>