mlx4_en: Coalescing target is equal for all mtu's

The interrupt moderation should not depend on number of incoming
bytes, but on number of incoming packets.
The previous scheme caused very high interrupts rate for small
messages when big MTU was configured.

Signed-off-by: Yevgeny Petrilin <yevgenyp@mellanox.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>