sfc: support the ethtool ksettings API properly so that 25/50/100G works

Store and handle ethtool link mode masks within the driver instead of
 just a single u32.  However, quite a significant amount of existing code
 wants to manipulate the masks directly, and thus now uses the first
 unsigned long (i.e. mask[0]) as though it were a legacy u32 mask.  This
 is ok because all the bits that code is interested in are in the first
 32 bits of the mask; but it might be a good idea to change them in
 future to use the proper bitmap API.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>