dwc2: flush fifo in dcd_edpt_close_all()

[HiFiPhile]

Describe the PR

Tested with net_lwip_webserver on STM32F407DISCO, with command:

 echo 1 | sudo tee /sys/bus/usb/devices/3-5/bConfigurationValue
 echo 2 | sudo tee /sys/bus/usb/devices/3-5/bConfigurationValue

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For labeler somebody said use pull_request_target
https://github.com/orgs/community/discussions/57878

[hathach]

thank you, I separate tx/rx flush to reuse part of the code. always inline would help to reduce overhead for calling 2 functions.

[hathach]

For labeler somebody said use pull_request_target
https://github.com/orgs/community/discussions/57878

ah thanks, trying to use that in #2591 to see if that works.