net: stmmac: Setup a default RX Coalesce value instead of the minimum

For performance reasons, sometimes using the minimum RX Coalesce value
is not optimal. Lets setup a default value that is optimal in most of
the use cases.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>