qualcommax: qca_edma: give the DSA conduit a stable MAC address

qualcommax: qca_edma: give the DSA conduit a stable MAC address

edma_probe() assigns a random address unconditionally, so the conduit
changes its MAC on every boot, and with it every DSA user port that has
no address of its own and therefore inherits the conduit's.

The conduit is a DMA engine behind the switch and has no address of its
own, which is why no board describes one for it. Take the address from
the switch this conduit serves instead: its ports carry the board's
addresses, either from DT or patched in by the bootloader, so this needs
nothing added per board. Keep the random address for the case where the
switch describes no address either.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Julius Bairaktaris <julius@bairaktaris.de>
Link: https://github.com/openwrt/openwrt/pull/24512
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>