net: ethernet: ti: cpsw: fix early budget split

The budget split function requires the phy speed to be known.
While ndo open a phy speed identification is postponed till the
moment link is up. Hence, move it to appropriate callback, when link
is up.

Reported-by: Grygorii Strashko <grygorii.strashko@ti.com>
Fixes: 8feb0a19 ("net: ethernet: ti: cpsw: split tx budget according between channels")
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>