cxgb4: Avoid a -Wflex-array-member-not-at-end warning

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

Move the conflicting declaration to the end of the structure. Notice
that `struct ethtool_dump` is a flexible structure --a structure that
contains a flexible-array member.

Fix the following warning:
./drivers/net/ethernet/chelsio/cxgb4/cxgb4.h:1215:29: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/Z6GBZ4brXYffLkt_@kspp


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