[POWERPC] iSeries: Fix unregistering HV event handlers

Commit fbd568a3 ("Change
synchronize_kernel to _rcu and _sched") changed the deprecated
synchronize_kernel() in HvLpEvent_unregisterHandler() to
synchronize_rcu().  It turns out that it should have been
synchronize_sched().

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>