Fix stm32f723disco host/cdc_msc_hid HIL: UART RX starvation + DWC2 DMA split-IN NAK storm (#3677)

Fix stm32f723disco host HIL: UART RX starvation + DWC2 split bulk NAK/XactErr handling (#3677)
stm32f7 BSP — UART RX starvation
- The host console USART shared interrupt priority with the USB OTG ISR, so a long
  OTG interrupt could starve RXNE and drop received bytes. Raise the USART RX IRQ
  above OTG_FS/OTG_HS in both the bare-metal and FreeRTOS init paths, guarded by
  #ifdef UART_ID so boards without a UART console keep the default OTG priority.

dwc2 host — split NAK/XactErr handling
- Slave mode: a persistently-NAKing split bulk/control IN poll re-armed the
  start-split immediately, storming the ISR and starving task context. Throttle by
  disabling the channel and re-arming on the resulting halt (no frame deferral).
- Buffer-DMA mode: a pure split bulk-OUT NAK was unhandled, leaving the channel
  halted and stalling the transfer — the dominant cause of CDC echo truncation.
  Handle it by rewinding the buffer pointers and retrying the start-split
  (Programming Guide v4.20a 5.1.4.2).
- Buffer-DMA mode: a split bulk-OUT XactErr was retried immediately, exhausting
  HCD_XFER_ERROR_MAX before the transient cleared. Throttle via channel_disable +
  re-arm to give the hub TT a recovery gap, mirroring slave mode.
- All three are scoped to split transfers (hcsplt.split_en); non-split NAK/XactErr
  keep the core-handled / immediate-retry behavior. The OUT XactErr throttle also
  excludes periodic split, where channel_disable() is a no-op and would wedge the
  channel. The nak_disabled flag is generalized to retry_disabled and honors
  xfer->closing so an endpoint close during a throttled retry tears down cleanly.

Verified on stm32f723disco HIL (slave + CFG_TUH_DWC2_DMA_ENABLE): host/cdc_msc_hid,
msc_file_explorer, and device_info all pass on both variants; DMA CDC echo went
from ~15-25% raw failure to 10/10 clean.