drivers: pwm: s/pwm/channel

All PWM API functions take a "pwm" parameter that indicates the selected
PWM channel. The variable name (pwm) and its documentation "PWM pin" is
misleading. This patch changes it to "channel" in line with the
terminology used in Devicetree (see DT_PWMS_CHANNEL... family of
macros).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>