powerpc: Fix PS3 allmodconfig warning

The struct ps3_notification_device in the ps3_probe_thread routine
is too large to be on the stack, causing a warning for an
allmodconfig build with clang.

Change the struct ps3_notification_device from a variable on the stack
to a dynamically allocated variable.

Reported-by: Arnd Bergmann <arnd@kernel.org>
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/d64f06f4-81ae-4ec5-ab3b-d7f7f091e0ac@infradead.org