net/mlx5e: hide an unused variable

The uplink_rpriv variable was added at the start of the function but
only used inside of an #ifdef:

drivers/net/ethernet/mellanox/mlx5/core/en_tc.c: In function 'mlx5e_route_lookup_ipv6':
drivers/net/ethernet/mellanox/mlx5/core/en_tc.c:1549:25: error: unused variable 'uplink_rpriv' [-Werror=unused-variable]

This moves the declaration into that #ifdef as well.

Fixes: 5ed99fb4 ("net/mlx5e: Move ethernet representors data into separate struct")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>