IB/iser: Return error to upper layers on EAGAIN registration failures

Commit 819a0873 ("IB/iser: Avoid error prints on EAGAIN
registration failures") not only eliminated the error print on that
case, but rather also modified the code such that it doesn't return
any error to upper layers.  As a result a wrong mapping was used.  Fix
this to correctly return the error in that case.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>