[bsp][gd32] fix pwm enable channel check

[Old-Ding]

Summary

• define the PWM channel number before validating it in the GD32 PWM enable path
• keep the enable/disable behavior unchanged and match the existing period/pulse helpers

Root cause

drv_pwm_enable() checks channel_num, but the local variable was never initialized in that function after the GD32 PWM refactor. The adjacent period and pulse helpers already derive it from configuration->channel before the same validation.

Closes #11331

Testing

• git diff --check
• verified the diff is limited to one insertion in bsp/gd32/arm/libraries/gd32_drivers/drv_pwm.c
• verified the file keeps CRLF line endings

Not run: GD32 BSP build. This machine does not have scons, gcc, arm-none-eabi-gcc, or clang installed.

[github-actions]

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[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• gd32105r-start

[CYFS3]

lgtm

[Rbb666]

ci-attach也麻烦添加下吧，避免后面出现类似情况。教程：https://club.rt-thread.org/ask/article/d273bbcd1f8779bc.html

[Old-Ding]

已补充 ci-attach：新增 bsp/gd32/arm/gd32405rg/.ci/attachconfig/ci.attachconfig.yml，打开 CONFIG_BSP_USING_PWM=y 和 CONFIG_BSP_USING_PWM0=y。

这个 BSP 已在 gd32_n32_apm32 CI 分组里，并且本身有 PWM Kconfig 声明；加 attachconfig 后会额外编译 GD32 PWM 驱动路径，后续类似 BSP_USING_PWM 打开后才暴露的问题可以被 CI 覆盖到。

本地已检查：

• git diff --check
• attachconfig YAML 可解析

新 CI 已由提交触发，正在跑。

[CYFS3]

已补充 ci-attach：新增 bsp/gd32/arm/gd32405rg/.ci/attachconfig/ci.attachconfig.yml，打开 CONFIG_BSP_USING_PWM=y 和 CONFIG_BSP_USING_PWM0=y。

这个 BSP 已在 gd32_n32_apm32 CI 分组里，并且本身有 PWM Kconfig 声明；加 attachconfig 后会额外编译 GD32 PWM 驱动路径，后续类似 BSP_USING_PWM 打开后才暴露的问题可以被 CI 覆盖到。

本地已检查：

• git diff --check
• attachconfig YAML 可解析

新 CI 已由提交触发，正在跑。

CI报错了麻烦处理一下

[Old-Ding]

更新一下 CI 结果：gd32_n32_apm32 的失败不是本 PR 这处 PWM channel 判断改动导致的编译错误，而是新增 ci-attach 打开 gd32405rg 的 BSP_USING_PWM/PWM0 后，暴露了 gd32-arm-series package 里的 F4xx timer 源文件依赖缺口。

失败日志里 drv_pwm.o 已经被编译，但链接阶段缺这些 GD32 标准外设 timer 符号：timer_deinit、timer_channel_output_config、timer_enable 等。根因是 package 的 GD32F4xx/SConscript 当前没有在 RT_USING_PWM/RT_USING_HWTIMER 下加入 GD32F4xx_standard_peripheral/Source/gd32f4xx_timer.c。

我已经在 package 仓库补了对应修复：RT-Thread-packages/gd32-arm-series#11。等这个 package PR 合入后，重新跑本 PR 的 BSP CI attach 应该就能覆盖并通过 gd32405rg PWM0 路径。