tcmu: Allow cmd_time_out to be set to zero (disabled)

The new cmd_time_out configfs attribute for TCMU is allowed to
be disabled, so go ahead and drop the tcmu_cmd_time_out_store()
check.

Reported-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>