net: mvpp2: adjust the coalescing parameters

This patch adjust the coalescing parameters to the vendor
recommendations for the PPv2 network controller.

Suggested-by: Yan Markman <ymarkman@marvell.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>