r8169: improve rtl_coalesce_choose_scale

The time limit provided by userspace is multiplied with 1000,
what could result in an overflow. Therefore change the time limit
parameter unit from ns to us, and avoid the problematic operation.
If there's no matching scale because provided time limit is too big,
return ERANGE instead of EINVAL to provide a hint to the user what's
wrong.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>