firewire: cdev: fix 32 bit userland on 64 bit kernel compat corner cases

Clemens points out that we need to use compat_ptr() in order to safely
cast from u64 to addresses of a 32-bit usermode client.

Before, our conversion went wrong
  - in practice if the client cast from pointer to integer such that
    sign-extension happened, (libraw1394 and libdc1394 at least were not
    doing that, IOW were not affected)
or
  - in theory on s390 (which doesn't have FireWire though) and on the
    tile architecture, regardless of what the client does.
The bug would usually be observed as the initial get_info ioctl failing
with "Bad address" (EFAULT).

Reported-by: Carl Karsten <carl@personnelware.com>
Reported-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>