sfc: set interrupt moderation via MCDI

SFN8000-series NICs require a new method of setting interrupt moderation,
via MCDI. This is indicated by a workaround flag. This new MCDI command
takes an explicit time value rather than a number of ticks. It therefore
makes sense to also store the moderation values in terms of time, since
that is what the ethtool interface is interested in.

Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>