scsi/scsi-disk: Remove redundant statement in scsi_disk_emulate_command()

Clang static code analyzer show warning:
scsi/scsi-disk.c:1918:5: warning: Value stored to 'buflen' is never read
    buflen = req->cmd.xfer;
    ^        ~~~~~~~~~~~~~

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200302130715.29440-7-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>