net: aquantia: Bad udp rate on default interrupt coalescing

Default Tx rates cause very long ISR delays on Tx.
0xff is 510us delay, giving only ~ 2000 interrupts per seconds for
Tx rings cleanup. With these settings udp tx rate was never higher than
~800Mbps on a single stream. Changing min delay to 0xF makes it
way better with ~6Gbps

TCP stream performance is almost unaffected by this change, since LSO
optimizations play important role.

CPU load is affected insignificantly by this change.

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>