net: ethernet: ti: cpsw_ethtool: allow res split while down

That's possible to set channel num while interfaces are down. When
interface gets up it should resplit budget. This resplit can happen
after phy is up but only if speed is changed, so should be set before
this, for this allow it to happen while changing number of channels,
when interfaces are down.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>