boards: nrf9160_pca10090: Fix polarity of pwm0/ch0

This is a follow up to commit 436c4262.
By default the channel 0 of pwm0 is set to the pin that drives led0.
Since the LED is active high, the inversion of polarity is incorrect.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>