ps3: Replace direct file operations by callback

Currently the FLASH database is updated by the kernel using file operations,
meant for userspace only. While this works for us because copy_{from,to}_user()
on powerpc can handle kernel pointers, this is unportable and a bad example.
Replace the file operations by callbacks, registered by the ps3flash driver.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Cc: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>