drivers: pwm: rpi_pico: Use “pico_channel” instead of “ch”

pwm_set_chan_level uses slice channels A(=0) or B(=1) and not Zephyr
channel (0..15). So PWM doesn't work for channels > 1. There is already
a function (pwm_rpi_channel_to_pico_channel) which does the right thing,
but it isn't used for pwm_set_chan_level.

Signed-off-by: Jan Hilsdorf <jan.hilsdorf@gmail.com>