powerpc: Fix spin_event_timeout() to be robust over context switches

Current implementation of spin_event_timeout can be interrupted by an
IRQ or context switch after testing the condition, but before checking
the timeout.  This can cause the loop to report a timeout when the
condition actually became true in the middle.

This patch adds one final check of the condition upon exit of the loop
if the last test of the condition was still false.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>