pwm: jz4740: Make disable operation compatible with TCU2 mode

On the JZ4750 and later SoCs, channel 1 and 2 operate in a different
way (TCU2 mode) as the other channels. If a TCU2 mode counter is
stopped before its PWM functionality is disabled, the output is not
guaranteed to return to the initial level.

Signed-off-by: Maarten ter Huurne <maarten@treewalker.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>