pwm: sifive: make set_config() and set_enable() work properly

The pwm_sifive_set_config() and pwm_sifive_set_enable() cannot work
properly due to the wrong implementations. It will cause the u-boot
PWM command to not work as expected. The bugs will be resolved in this
patch.

Signed-off-by: Vincent Chen <vincent.chen@sifive.com>
Reviewed-by: Rick Chen <rick@andestech.com>