target/user: Fix a data type in tcmu_queue_cmd()

This patch avoids that sparse reports the following error messages:

drivers/target/target_core_user.c:547:13: warning: incorrect type in assignment (different base types)
drivers/target/target_core_user.c:547:13:    expected int [signed] ret
drivers/target/target_core_user.c:547:13:    got restricted sense_reason_t
drivers/target/target_core_user.c:548:20: warning: restricted sense_reason_t degrades to integer
drivers/target/target_core_user.c:557:16: warning: incorrect type in return expression (different base types)
drivers/target/target_core_user.c:557:16:    expected restricted sense_reason_t
drivers/target/target_core_user.c:557:16:    got int [signed] ret

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>