net: dsa: yt921x: Fix parsing MIB attributes

net: dsa: yt921x: Fix parsing MIB attributes

There are hard-to-find unused fields in the MIB table I didn't notice in
the example driver code, causing wrong interpretation of the MIB data.

For some 64-bit attributes, the current (wrong) implementation took the
correct lower 32 bits, but messed up the upper 32 bits, so it would work
accidentally until 32-bit overflows happen. Fix that too.

Fixes: 186623f4aa72 ("net: dsa: yt921x: Add support for Motorcomm YT921x")
Signed-off-by: David Yang <mmyangfl@gmail.com>
Link: https://patch.msgid.link/20251126084024.2843851-2-mmyangfl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>