net: ethernet: ti: cpsw: optimize end of poll cycle

Check budget fullness only after it's updated and update
channel mask only once to keep budget balance between channels.
It's also needed for farther changes.

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