net/mlx5e: reduce stack usage in mlx5_eswitch_termtbl_create

Putting an empty 'mlx5_flow_spec' structure on the stack is a bit
wasteful and causes a warning on 32-bit architectures when building
with clang -fsanitize-coverage:

drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c: In function 'mlx5_eswitch_termtbl_create':
drivers/net/ethernet/mellanox/mlx5/core/eswitch_offloads_termtbl.c:90:1: error: the frame size of 1032 bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

Since the structure is never written to, we can statically allocate
it to avoid the stack usage. To be on the safe side, mark all
subsequent function arguments that we pass it into as 'const'
as well.

Fixes: 10caabda ("net/mlx5e: Use termination table for VLAN push actions")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>