net: aquantia: tx clean budget logic error

In case no other traffic happening on the ring, full tx cleanup
may not be completed. That may cause socket buffer to overflow
and tx traffic to stuck until next activity on the ring happens.

This is due to logic error in budget variable decrementor.
Variable is compared with zero, and then post decremented,
causing it to become MAX_INT. Solution is remove decrementor
from the `for` statement and rewrite it in a clear way.

Fixes: b647d398

 ("net: aquantia: Add tx clean budget and valid budget handling logic")
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>