hidp: fix compat_ioctl

1) no point putting it into fs/compat_ioctl.c when you handle it in
your ->compat_ioctl() anyway.
2) HIDPCONNADD is *not* COMPATIBLE_IOCTL() stuff at all - it does
layout massage (pointer-chasing there)
3) use compat_ptr()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>