net: aquantia: Enable coalescing management via ethtool interface

Aquantia NIC allows both TX and RX interrupt throttle rate (ITR)
management, but this was used in a very limited way via predefined
values. This patch allows to setup ITR default values via module
command line arguments and via standard ethtool coalescing settings.

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>