eth: sundance: fix endian issues

Fix sparse warnings about endianness. Store DMA addr to a variable
of correct type and then only convert it when writing to the descriptor.

Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250901210818.1025316-2-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>