net: ethtool: Avoid thousands of -Wflex-array-member-not-at-end warnings

net: ethtool: Avoid thousands of -Wflex-array-member-not-at-end warnings

-Wflex-array-member-not-at-end was introduced in GCC-14, and we are
getting ready to enable it, globally.

Change the type of the middle struct member currently causing trouble from
`struct ethtool_link_settings` to `struct ethtool_link_settings_hdr`.

Additionally, update the type of some variables in various functions that
don't access the flexible-array member, changing them to the newly created
`struct ethtool_link_settings_hdr`. These changes are needed because the
type of the conflicting middle members changed. So, those instances that
expect the type to be `struct ethtool_link_settings` should be adjusted to
the newly created type `struct ethtool_link_settings_hdr`.

Also, adjust variable declarations to follow the reverse xmas tree
convention.

Fix 3338 of the following -Wflex-array-member-not-at-end warnings:

include/linux/ethtool.h:214:38: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/0bc2809fe2a6c11dd4c8a9a10d9bd65cccdb559b.1730238285.git.gustavoars@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>