net/mlx5e: Reduce stack use reading PCIe congestion thresholds

net/mlx5e: Reduce stack use reading PCIe congestion thresholds

union devlink_param_value grew when U64 array parameters were added.
Keeping union devlink_param_value val[4] in
mlx5e_pcie_cong_get_thresh_config() exceeded the compiler's
-Wframe-larger-than limit.

Reuse one union: call devl_param_driverinit_value_get() once per
MLX5 PCIe congestion threshold and assign each vu16 to the
corresponding mlx5e_pcie_cong_thresh member.

Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Ratheesh Kannoth <rkannoth@marvell.com>
Link: https://patch.msgid.link/20260521095303.2395584-3-rkannoth@marvell.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>