gve: Add tx|rx-coalesce-usec for DQO

Adding ethtool support for changing rx-coalesce-usec and tx-coalesce-usec
when using the DQO queue format.

Signed-off-by: Tao Liu <xliutaox@google.com>
Signed-off-by: Jeroen de Borst <jeroendb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>