mlxsw: spectrum_span: Use a more fitting error code

ENOENT is suitable when an item is looked for in a collection and can't
be found. The failure here is actually a depletion of a resource, where
ENOBUFS is the more fitting error code.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>