pwm: pwm-tiecap: Disable APWM mode after configure

APWM mode is enabled while configuring PWM device. This was done to
handle shadow & immediate mode update of period and compare registers.
However, leaving it enabled after configuring will cause APWM output on
PWM pin even before enabling PWM device.
Fix the same by disabling APWM mode after configuring if PWM device is
not running.

Signed-off-by: Philip, Avinash <avinashphilip@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>