sidecar release

stm32h7-eth: Properly maintain Tx tail pointer

Analagous to #1493, we should maintain the Tx tail pointer
properly, as well.  Note that since the device won't examine a
Tx descriptor until the tail pointer is set appropriately, we
can relax stores and rely on the barrier in `tx_notify` to flush
everything to RAM properly before writing the tail pointer and
kicking off a DMA.