mlxsw: spectrum_router: remove redundant initialization of pointer br_dev

The pointer br_dev is being initialized with a value that is never read
and is being updated with a new value later on. The initialization
is redundant and can be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>