drivers/ieee1394/raw1394.c: fix a NULL pointer

The coverity checker spotted that this was a NULL pointer dereference in
the "if (copy_from_user(...))" case since the next step is to
kfree(cache->filled_head).

There's no need to free cache at this point, and it's getting free'd
later.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Jody McIntyre <scjody@modernduck.com>