staging: vt6655: fix sparse warning "cast removes address space of expression"

Add missing __user macro to second parameter of ethtool_ioctl().
This removes the need for the offending (void *) cast of the user space pointer
rq->ifr_data.

Signed-off-by: James A Shackleford <shack@linux.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>