realtek: eth: avoid TX buffer memory leak

realtek: eth: avoid TX buffer memory leak

Although never observed, a transmit timeout might happen.
In that case there is a resource leak inside rteth_tx_timeout().
This happens when rteth_setup_ring_buffer() reinitializes the
transmit buffers and overwrites all transmit slots. Any linked
SKB is lost and leaked at this point.

Be defensive and add a cleanup rteth_free_tx_buffers() function.
Call this alongside rteth_free_rx_buffers() where needed.

Link: https://github.com/openwrt/openwrt/pull/23483
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>