mlxsw: spectrum: Use notifier_from_errno() in notifier block

Instead of checking the error value and returning NOTIFY_BAD, just use
notifier_from_errno() 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>