dwc2: cleanup setup_packet pointer cast (review feedback)

Cast DOEPDMA0 through uintptr_t and use sizeof(tusb_control_request_t)
instead of the magic constant 8, matching project convention. Add a
reference to Programming Guide v4.20a 9.1.2.1 for the DOEPDMAn-8 rule.

Addresses Copilot review comment; no functional change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>