[POWERPC] ps3: always make sure were running on a PS3

Add missing checks to PS3 specific drivers ps3av and sys-manager to verify that
we are actually running on a PS3 (pointed out by Arnd).

Correct existing checks in other subsystems/drivers to return -ENODEV instead
of zero.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>