drivers: display_nrf_led_matrix: Correct calculation of pixel period

This is a follow-up to commit 4dfab40c.

The calculation of the pixel period is done incorrectly and results
in higher than configured refresh frequency if pixels are refreshed
in groups. Fix that and lower the base frequency for the timer and
PWM so that wider range of refresh frequency can be used also with
the pixel grouping.

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