net: ethernet: ti: cpsw: combine budget and weight split and check

Re-split weight along with budget. It simplify code a little
and update state after every rate change. Also it's necessarily
to move arguments checks to this combined function. Replace
maximum rate check for an interface on maximum possible rate.

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