Please backport "net: lantiq: Add locking for TX DMA channel"

Hi,

Please backport commit f9317ae5523f99999fb54c513ebabbb2bc887ddf ("net: lantiq: Add locking for TX DMA channel") to kernel 5.4. https://git.kernel.org/linus/f9317ae5523f99999fb54c513ebabbb2bc887ddf

The fix commit was added upstream with kernel 5.9 and fixes a problem introduced in commit fe1a56420cf2 ("net: lantiq: Add Lantiq / Intel VRX200 Ethernet driver") with kernel 4.20.

Multiple users reported in the ticket to integrate this into OpenWrt that this fixes TX hangs for them. https://github.com/openwrt/openwrt/pull/3085

Hauke