pwm: omap-dmtimer: Fix PWM enabling sequence

To configure DM timer in PWM mode the following needs to be set in
OMAP_TIMER_CTRL_REG using set_pwm callback:
- Set toggle mode on PORTIMERPWM output pin
- Set trigger on overflow and match on PORTIMERPWM output pin.
- Set auto reload

This is a one time configuration and needs to be set before the start of
the DM timer. But the current driver tries to set the same configuration
for every period/duty cycle update, which is not needed. So move the PWM
setup before enabling timer and do not update it in
pwm_omap_dmtimer_config().

Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>