net: hibmcge: fix wrong mtu log issue

A dbg log is generated when the driver modifies the MTU,
which is expected to trace the change of the MTU.

However, the log is recorded after WRITE_ONCE().
At this time, netdev->mtu has been changed to the new value.
As a result, netdev->mtu is the same as new_mtu.

This patch modifies the log location and records logs before WRITE_ONCE().

Fixes: ff4edac6 ("net: hibmcge: Implement some .ndo functions")
Signed-off-by: Jijie Shao <shaojijie@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250410021327.590362-5-shaojijie@huawei.com


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