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 <[email protected]>
Reviewed-by: Petr Machata <[email protected]>
Reviewed-by: Przemek Kitszel <[email protected]>
Tested-by: Petr Machata <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Paolo Abeni <[email protected]>