mlxsw: spectrum_router: Update prefix count for IPv6

mlxsw: spectrum_router: Update prefix count for IPv6

The number of possible prefix lengths for IPv6 is 129 and not 128.

Fixes following warning from UBSAN when /128 routes are offloaded:

 UBSAN: Undefined behaviour in
drivers/net/ethernet/mellanox/mlxsw/spectrum_router.c:2510:27 index 128 is out
of range for type 'long unsigned int [128]'

Fixes: 5e9c16cc83a7 ("mlxsw: spectrum_router: Implement private fib")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>