rt2x00: use simple_read_from_buffer()

The problem with this copy_to_user() calls is that they don't ensure
that "size" is less than the "length" which the user provided.

Obviously, this is debugfs and "size" is normally going to be very small
so it probably doesn't matter, but this is the correct thing to do.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>