ethtool: Name the 'no change' value for setting RSS hash key but not indir table

We usually allocate special values of u32 fields starting from the top
down, so also change the value to 0xffffffff.  As these operations
haven't been included in a stable release yet, it's not too late to
change.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>