ethtool: fix sparse warning

Fixes sparse complaints about dropping __user in casts.
 warning: cast removes address space of expression

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>