stmmac: get/set coalesce parameters via ethtool

This patch is to get/set the tx/rx coalesce parameters
via ethtool interface.

Tests have been done on several platform with different GMAC chips w/o and w/
RX watchdog feature.

V2: reject coalesce settings that are not supported.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>