[POWERPC] Fix ppc64's writing to struct file_operations

In preparation for marking file_operations as const.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>