rt-thread [GD32_ARM] Fix bugs in drv_adc when enable multiple channel.

拉取/合并请求描述：(PR description)

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Test on GD32F303

ADC driver of GD32_ARM have multiple bugs.

ADC should use single operation mode, but current version use discontinuous operation mode.
When calibration is used after turning on the ADC, it is necessary to wait for 14 CK_ADC(about 350ns in GD32F303).
When enabling multiple ADC channels at once and sequentially reading the different channels, the values read are all the same.

Make sure ADC enable single operation mode.
Add a 1us dely after ADCON.
Clear EOC flag before start ADC convert to prevent other bugs.
The setting of the regular channel should be done at each reading, rather than at the point of enabling.

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[ ] 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
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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
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[ ] 代码是高质量的 Code in this PR is of high quality
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