powerpc/qe: Fixup QE_General4 errata

QE_General4 should only round up the divisor iff divisor is > 3.
Rounding up lower divisors makes the error too big, causing USB
on MPC832x to fail.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Acked-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>