drivers: pwm: rpi_pico: fix setting of cycle count per period

pwm_set_wrap() sets the TOP value, not the number of cycles.
Counter will run from 0 to TOP inclusive, generating TOP + 1 cycles.
To get n cycles, we need to set TOP to (n - 1).

The wrong setting made it impossible to achieve 100 % duty cycle, as
there was always one extra cycle.

Fixes: 7e0fff24 ("drivers: pwm: add pwm driver for rpi_pico")
Signed-off-by: Oliver Barta <o.barta89@gmail.com>