ib_srpt: wait_for_completion_timeout does not return negative status

This patch changes srpt_close_session() to properly use an unsigned long
for wait_for_completion_timeout()'s return value, and to update WARN_ON()
to only trigger for a zero return.

Signed-off-by: Nicholas Mc Guire <der.herr@hofr.at>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>