Update dfs_tmpfs.c

[meng-plus]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

#7887 (comment)

dfs_tmpfs.c			
Error[Pe513]: a value of type "dev_t" cannot be assigned to an entity of type "struct rt_device *"	E:\WorkSpaces\RT-Thread\rt-thread\components\dfs\dfs_v1\filesystems\tmpfs\dfs_tmpfs.c	488	

你的解决方案是什么 (what is your solution)

st->st_dev = (rt_device_t)dfs_filesystem_lookup(fs->path);

在什么测试环境下测试通过 (what is the test environment)

iar9 类型匹配通过上下文翻阅也是正常的

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用[formatting](https://github.com/mysterywolf/formatting) 等源码格式化工具确保格式符合[RT-Thread代码规范](https://github.com/RT-Thread/rt-thread/blob/master/documentation/contribution_guide/coding_style_cn.md) This PR complies with [RT-Thread code specification](https://github.com/RT-Thread/rt-thread/blob/master/documentation/contribution_guide/coding_style_en.md)