scsi: target: tcmu: fix error resetting qfull_time_out to default

Problem:

$ cat /sys/kernel/config/target/core/user_0/block/attrib/qfull_time_out
-1

$ echo "-1" > /sys/kernel/config/target/core/user_0/block/attrib/qfull_time_out
-bash: echo: write error: Invalid argument

Fix:

This patch will help reset qfull_time_out to its default
i.e. qfull_time_out=-1.

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>
Acked-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>