drivers: pwm_nrf5_sw: Correctly use allocated GPIOTE channels

The driver improperly uses the PWM channel index to reference
the GPIOTE channel to be used for the PWM signal generation.
Consequently, the PWM signal on a given channel can be correctly
generated only if both those indexes are by chance the same.
Fix this by switching to use the stored index of the actually
allocated GPIOTE channel.

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