mlxsw: spectrum_router: Fix use of uninitialized adjacency index

When mlxsw_sp_adj_discard_write() is called for the first time, the
value stored in 'mlxsw_sp->router->adj_discard_index' is invalid, as
indicated by 'mlxsw_sp->router->adj_discard_index_valid' being set to
'false'.

In this case, we should not use the value initially stored in
'mlxsw_sp->router->adj_discard_index' (0) and instead use the value
allocated later in the function.

Fixes: 983db619

 ("mlxsw: spectrum_router: Allocate discard adjacency entry when needed")
Signed-off-by: Amit Cohen <amitc@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>