samples: basic: blinky_pwm: fix divider setting for rpi_pico

The fractional part of the divider value is a 4 bit value. Setting it to
255 leads to an overflow in pwm_rpi_get_clkdiv(). This has resulted in a
slight deviation from the reported timing, which is btw. printed in nsec.

Fixes: c5cb0d1a ("samples: blinky_pwm: Fix sample for rpi_pico")
Signed-off-by: Oliver Barta <o.barta89@gmail.com>