[BSP][stm32] fix the compile error

[ZosCat1]

拉取/合并请求描述：(PR description)

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请在这里填写您的PR描述，可以包括以下之一的内容：为什么提交这份PR；解决的问题是什么，你的解决方案是什么；
Please fill in your PR description here, which can include one of the following items: why to submit this PR; what is the problem solved and what is your solution;
优化RTduino一键兼容驱动adc报错问题

并确认并列出已经在什么情况或板卡上进行了测试。
And confirm in which case or board has been tested. -->
stm32f103c8t6

为什么提交这份PR (why to submit this PR)

优化RTduino一键兼容驱动adc报错问题

你的解决方案是什么 (what is your solution)

修改drv_adc.c文件

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

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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等源码格式化工具确保格式符合RT-Thread代码规范 This PR complies with RT-Thread code specification