net: stmmac: selftests: Update status when disabling RSS

We are disabling RSS on HW but not updating the internal private status
to the 'disabled' state. This is needed for next tc commit that will
check if RSS is disabled before trying to apply filters.

Fixes: 4647e021 ("net: stmmac: selftests: Add selftest for L3/L4 Filters")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>