clocksource: sh_tmu: Make undefined TCOR behaviour less undefined.

Avoid undocumented vague TMU behavior when zero value is set to TCOR.

This primarily fixes up issues encountered under qemu with a zero-length
period, while the hardware itself is fairly ambivalent one way or the
other.

Signed-off-by: Shin-ichiro KAWASAKI <kawasaki@juno.dti.ne.jp>
Acked-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>