lan743x: fix endianness when accessing descriptors

[ Upstream commit 46251282

 ]

TX/RX descriptor ring fields are always little-endian, but conversion
wasn't performed for big-endian CPUs, so the driver failed to work.

This patch makes the driver work on big-endian CPUs. It was tested and
confirmed to work on NXP P1010 processor (PowerPC).

Signed-off-by: Alexey Denisov <rtgbnm@gmail.com>
Link: https://lore.kernel.org/r/20210128044859.280219-1-rtgbnm@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>