mlxsw: spectrum_router: fix xa_store() error checking

mlxsw: spectrum_router: fix xa_store() error checking

It is meant to use xa_err() to extract the error encoded in the return
value of xa_store().

Fixes: 44c2fbebe18a ("mlxsw: spectrum_router: Share nexthop counters in resilient groups")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Tested-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20241017023223.74180-1-yuancan@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>