amd-xgbe: add TX descriptor cleanup for link-down

amd-xgbe: add TX descriptor cleanup for link-down

Add intelligent TX descriptor cleanup mechanism to reclaim abandoned
descriptors when the physical link goes down.

When the link goes down while TX packets are in-flight, the hardware
stops processing descriptors with the OWN bit still set. The current
driver waits indefinitely for these descriptors to complete, which
never happens. This causes:

  - TX ring exhaustion (no descriptors available for new packets)
  - Memory leaks (skbs never freed)
  - DMA mapping leaks (mappings never unmapped)
  - Network stack backpressure buildup

Add force-cleanup mechanism in xgbe_tx_poll() that detects link-down
state and reclaims abandoned descriptors. The helper functions and DMA
optimizations support efficient TX shutdown:
  - xgbe_wait_for_dma_tx_complete(): Wait for DMA completion with
    link-down optimization
  - Restructure xgbe_disable_tx() for proper shutdown sequence

Implementation:
  1. Check link state at the start of tx_poll
  2. If link is down, set force_cleanup flag
  3. For descriptors that hardware hasn't completed (!tx_complete):
     - If force_cleanup: treat as completed and reclaim resources
     - If link up: break and wait for hardware (normal behavior)

The cleanup process:
  - Frees skbs that will never be transmitted
  - Unmaps DMA mappings
  - Resets descriptors for reuse
  - Does NOT count as successful transmission (correct statistics)

Benefits:
  - Prevents TX ring starvation
  - Eliminates memory and DMA mapping leaks
  - Enables fast link recovery when link comes back up
  - Critical for link aggregation failover scenarios

Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Link: https://patch.msgid.link/20260319163251.1808611-4-Raju.Rangoju@amd.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>