mlxsw: spectrum: Use WARN_ON() return value

Instead of checking for a condition and then issue the warning, just do
it in one go and simplify the code.

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>