igc: use EOPNOTSUPP instead of ENOTSUPP in igc_ethtool_get_sset_count()

igc_ethtool_get_sset_count() returns -ENOTSUPP when a given stringset is
not supported, causing userland programs to get "Unknown error 524".

Since EOPNOTSUPP should be used when error is propagated to userland,
return -EOPNOTSUPP instead of -ENOTSUPP.

Fixes: 36b9fea6 ("igc: Add support for statistics")
Signed-off-by: Kohei Enju <enjuk@amazon.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>