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

cxgb4: Avoid -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.

Change the type of the middle struct member currently causing
trouble from `struct tc_u32_sel` to `struct tc_u32_sel_hdr`.

Fix the following warning:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_tc_u32_parse.h:245:27: 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/97388e8a7990975aa56cf0ada211764c735c3432.1723586870.git.gustavoars@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>