sfc: Use correct fields of struct ethtool_coalesce

An earlier developer misunderstood the meaning of the 'irq' fields and
the driver did not support the standard fields.  To avoid invalidating
existing user documentation, we report and accept changes through
either the standard or 'irq' fields.  If both are changed at the same
time, we prefer the standard field.

Also explain why we don't currently use the 'max_frames' fields.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>