powerpc: Fix bug in timebase synchronization on 32-bit SMP powermac

We were using udelay in the loop on the primary cpu waiting for the
secondary cpu to take the timebase value.  Unfortunately now that
udelay uses the timebase, and the timebase is stopped at this point,
the udelay never terminated.  This fixes it by not using udelay, and
increases the number of loops before we time out to compensate.

Signed-off-by: Paul Mackerras <paulus@samba.org>