IB/ehca: Fix error return code in ehca_create_slab_caches()

Fix to return -ENOMEM in the kmem_cache_create() error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Roland Dreier <roland@purestorage.com>