fix USB-PD implementation

[NyxCode]

Hey! This PR fixes usbpd.h in examples_x035/usbpd_sink.
What I wanted to get working was just a simple PD sink requesting 12V or 9V from a charger, but no chargers successfully negotiated.
With these fixes, negotiation works. I tested this on an iPad Pro charger and a ThinkPad T14 power brick.

Disclosure: This PR was assisted by Claude Code. Should be reviewed by someone familiar with the PD protocol.

[NyxCode]

Without the barrier (and the volatile field above), negotiation always failed when compiled with LTO

[cnlohr]

Got it.

[NyxCode]

The ThinkPad power brick sent the WAIT command, and only negotiated successfully with this

[cnlohr]

Got it.

[NyxCode]

For reference, here is my code that negotiates 12V or 9V:

#define NEGOTIATE_TIMEOUT_CYCLES \
    ((uint32_t)((uint64_t)FUNCONF_SYSTEM_CORE_CLOCK * 10000 / 1000))
#define SETTLE_MS 500

// kept around for diagnostics
static uint8_t s_last_state;

int32_t usb_pd_negotiate(void) {
    USBPD_Init(eUSBPD_VCC_3V3);
    USBPD_Reset();

    uint32_t start = (uint32_t)SysTick->CNTL;
    USBPD_Result_e result;
    while ((result = USBPD_SinkNegotiate()) == eUSBPD_BUSY) {
        if ((uint32_t)SysTick->CNTL - start >= NEGOTIATE_TIMEOUT_CYCLES) {
            s_last_state = (uint8_t)USBPD_GetState();
            return -1;
        }
    }
    s_last_state = (uint8_t)USBPD_GetState();
    if (result != eUSBPD_OK)
        return -2;

    USBPD_SPR_CapabilitiesMessage_t *caps;
    size_t n = USBPD_GetCapabilities(&caps);

    int     best_idx = -1;
    int32_t best_mv  = 0;
    for (size_t i = 0; i < n; i++) {
        const USBPD_SourcePDO_t *pdo = &caps->Source[i];
        if (pdo->Header.PDOType != eUSBPD_PDO_FIXED) continue;
        int32_t mv = (int32_t)(pdo->FixedSupply.VoltageIn50mV * 50u);
        if ((mv == 12000 || mv == 9000) && mv > best_mv) {
            best_mv  = mv;
            best_idx = (int)i;
        }
    }

    if (best_idx < 0)
        return -(3 + (int32_t)n);

    USBPD_SelectPDO((uint8_t)best_idx, 0);
    Delay_Ms(SETTLE_MS);
    return best_mv;
}