IB/ehca: Return proper error code if register_mr fails

Set the return code of ehca_register_mr() to ENOMEM if the corresponding
firmware call fails due to out of resources.  Some other error codes
were explicitly mapped to EINVAL -- just remove those cases so they
get mapped to the default case, which already returns EINVAL anyway.

Signed-off-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>