rockchip: pwm: Fix default polarity

In the code, the default polarity is set to positive/positive,
which is neither normal polarity or inverted polarity. It's
only the hardware default. This leads to booting linux with
wrong polarity setting.

Update the code to use PWM_DUTY_POSTIVE | PWM_INACTIVE_NEGATIVE
by default instead.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>